Ok, I did a bit more testing on this with a revelation I had...
The Bow+Arrow combo is "bugged" in FF4A... I could be wrong if it is a bug, but I'll call it that anyways.
I originally thought Rosa's Attack command is tied to some character specific code that cripples her usage of the Bow+Arrow combo, to force the use of Aim.
However, that isn't the case.
Apparently, the back row bit is ignored when a Bow+Arrow combo is equipped.
All Bows and Arrows have the back row bit enabled. When testing this in battle, it appears that being in the front row does make a difference.. as full hit rate is available. In the back row, approximately half of the damage is dealt (which means 1/2 hit rate penalty is applied).
So, in conclusion:
To deal full damage with the Bow+Arrow combo in FF4A, you need to one of the following:
1) Have the Aim command, so you can use it effectively in the back row.
2) Be in the front row.
This isn't Rosa specific, but this is how the Bow+Arrow combo is coded.