The whole premise is really simple. Jump makes Kain that much more powerful than Cecil.
It has been been argued before that Jump takes a lot of time. It's true, to an extent.
There are many factors that assist Jump to the point where Kain isn't being missed too much.. at worst, he's "realigning" himself with the rest of group a tad late.
1) Kain's Agility (which Jump also relies on) is naturally higher than Cecil.
2) Kain's other offensive stats are also ahead of Cecil (and advances quicker than Cecil's Agility late game, post level 70)
3) Even at optimized Agility (99, via a Crystal Ring, having 96 Agility naturally), he's still fast enough to catch up back to his own turn (although, that's not a given, but it can happen at faster battle speeds)...
Hopefully the movies don't desync -
http://rapidshare.com/files/148687170/ff4movies.zip.html 
You will need J2e's translation, though, in theory it shouldn't affect stuff w/o the patch, but it might happen.