I did some thinking about this piece of equipment, and tried to be critical on the both sides of the issue.
I've concluded that this piece of armor is underrated, and perhaps undervalued by most people (and you could've figure that from the title).
Let's look deeper into the details and the women that can equip this robe.
Here's a look at some end game armor...
Heroine/Minerva Robe:
20 defense
40% evade
5 magic defense
5% magic evade
+15 Str, Agi, Vit
-15 Wis, Will
resistant vs paralysis
White Robe:
18 defense
10% evade
10 magic defense
6% magic evade
+15 Will
resistant vs blind
resistant against spirits and undead
Sorceror/Light Robe:
12 defense
10% evade
9 magic defense
9% magic evade
+5 Wis, Will
resistant vs lightning
resistant against undead
I'm not going to include Porom's FF4A's Lord's Robe because it's actually the best combo with the Hero Shield (in terms of overpoweredness).
On the pure basis that Rydia could equip the White Robe if she was still the Younger version... it would make this an interesting argument, but let's not worry about that detail too much (Rydia's best equipment in FF4A is overpowered anyways).
If you look over the equipment, they are evenly matched in different aspects. The Heroine Robe primarily converts a decent spellcaster into more of a fighting machine. The White Robe provides a nice compromise of defense increases plus Will Power boosts with decent resistances (so Rosa/Porom will be 100% effective for healing). The Sorceror's Robe provides the best balance of magic evasion and magic boosts which tends to help (Younger) Rydia realize her potential.
There are a few things that are overrated about some of the equipment. Here's some point worth pondering:
The White Robe has resistances vs blind... but that's useless since no monster in the Lunar Subterrain use a blind attack. Even worse is that there are no Spirit monsters past the Tower of Babil (heading towards the underground) and that the undead you actually meet are virtually harmless while in the back row. Sure they poison you, but that's because you have a lower level than they do! Also consider this.. if you are maxing Rosa or Porom's (or even Rydia) stats to level 99, this stat boost is useless. This isn't quite applicable for those that don't level/stat max.
The Sorceror's Robe is the weakest of your defense options... 10% evade means that you will require Cecil to cover her (either by wasting a turn or letting him be the physical damage shield). Unless you are using FF4A's Hero Shield (or not abusing the Shield trick), the girls will stay relatively frail physically.
The Heroine's Robe has the weakest magic defense and naturally remove a magic evade multiplier because of the reduction in both magic stats. However, there are outside factors when you consider the characters that can use it... so let's analyze the girls, shall we?
Rydia:
Best stat growth options past level 70:
+3 Str, Agi, Wis, Will
+2 Str, Vit, Wis, Will
If you plan on making her stronger , the Heroine Robe can serve that purpose. This becomes less effective over time as her important stats can be gained at a far greater rate than Vitality, which has the least impact. The Heroine Robe still provides excellent protection for defense, even though her magic defense multiplier (and magic power) can take a nose dive.. but that can be countered by using the classic Tiara (Gold Hairpin).. which does benefit Rydia the most since Wisdom powers most magic. However, let's put this in perspective.
Let's assume Rydia has a setup similar to this:
Note: These are legitmate possible final stats.
(base stats)
level 99
Str 99
Agi 96
Vit 50
Wis 99
Will 99
19x attack multiplier
12x defense multiplier
9x magic defense multiplier
With the equipment set:
Dragon Whip
Golden Hairpin
Heroine Robe
Protect Ring
The stats come out like this..
@ level 99
Str 99
Agi 99
Vit 85
Wis 89
Will 79
19x 103 attack, 99% hit rate
12x 79 defense, 65% evade
8x 27 magic defense, 45% magic evade
Ok, so you've lost a magic defense multiplier and some magic power, but you still have gained the following..
Resistance vs Dragons and a crapload of evade.
You lose a spell multiplier of 2x in the process, but you may be surprised that you aren't missing that much when it comes to dealing damage.
Meteo will do 9999 damage regardless of the monster's magic defense past a certain point (89 Wisdom is way above than you'll ever need for this spell), so that's not weakened.
Flare will not consistantly do 9999 damage on enemies with virtually magic defense (Flare has a spell base power of 400.. so when you consider 25 to be the maximum, it's not that great a loss). Flare will still do mediocre vs monsters with high magic defense anyways.
Bahamut will do a bit better (since it has a spell base power of 480) and won't ever consistantly do 9999 damage on Behemoths either.
I could probably compute the numbers, but the difference is like at most ~1000 points of damage. If you're really THAT concerned, just drop the whip for the Stardust Rod anyways, and you don't miss that much to begin with!
With another character, such as Rosa (and/or Porom).. there's a bigger discrepancy...
Rosa:
Best stat growth options past level 70:
+1 all
+2 Str, Will
+2 Str, Wis, Will (Porom only)
Rosa's final stats are superior to Porom's for the most part (primarily for Str and Vit).
Rosa's final growth really sucks, and needs massive help... plus her magic defense growth is slower than molasses. The same idea can be done for Rosa like it did for Rydia, but with even greater benefits...
(base stats)
@ level 99
81 Str
57 Agi
67 Vit
47 Wis
99 Will
14x attack multiplier
7x defense multiplier
5x magic defense multiplier
With the equipment set:
Artemis Arrows
Elven Bow
Golden Hairpin
Heroine Robe
Protect Ring
The stats come out like this..
@ level 99
Str 96
Agi 72
Vit 97
Wis 47
Will 84
17x 124 attack, 99% hit rate
9x 85 defense, 65% evade
6x 27 magic defense, 40% magic evade
In addition to having an increased magic defense multiplier (there is no way to do this w/o increasing the Agility and compensating for the losses to Wisdom.. you also don't want to trade the Protect Ring for the Rune Ring for elemental resistance reasons), she gets the additional benefit of killing dragons and mages just as effectively while being decently fast (not great, but better than shit).
The most important part of Rosa's role is healing, and in looking at it closely, she does lose some healing potentency.
3 * 288 (Cure4's base power) * 1 (worst case) / 5 (split damage) = 172
3 * 288 (Cure4's base power) * 3/2 (best case) / 5 (split damage) = 259
So, here's the tradeoff... lose potentially 172-259 points of healing, or have an above average Rosa who is decently fast, can kick ass (she'd be slightly better than Rydia), becomes very versatile in killing the majority of the monsters in the Lunar Subterrain (and subsequently can kick ass in the Lunar Ruins from FF4A), gets resistance to dragon attacks, evade lots of physical damage, and gets the extra magic defense point to boot.
So, I rest my case. Please feel free to question to debate the differences in detail, but if you think the Heroine/Minvera Robe sucks, you haven't really looked into its effectiveness in the game. The results will shock and surprise you nicely.
Note that magic defense is somewhat overrated given what monsters use on you... the magic defense multiplier has a significantly greater impact in damage taking due to magic defense in this game (even though it is countered somewhat in FF4A, its impact is not that great when you consider what my magic damage algo says).