Porom maybe but the final boss counters magic attacks, no? In the GBA version you could switch party members but Rydia and Palom aren't really useful since the boss counter their spells. In the DS version he also counters summons too.
Two black mages would be great for the final dungeon, though. Swapping someone else into his slot would be preferable to some for the final boss (though that would mean trekking through the dungeon again).
But, since I like having the twins in the final battle party, sometimes you have to take the damage to dish it. As long as you play defensively with your buffs you should be fine. Actually, having two white mages to double-up on the buffing would be beneficial. In the SNES version, and coincidentally as we speak I'm going through the two ways to beat the final boss, he certainly does counter everything. But it's moreso when he switches into Phase 2/3/4. I feel like fighting him using the Second Form Glitch is a little harder because you have to keep track of a few more factors than Big Bang.
Swapping out Kain for someone like Yang would be nice. Having swords to throw + using some of the focus attacks would be more beneficial than jumping. But then you lose the advantage of having a party member that can't be hit if you use a command. Almost every member has his/her benefits, and since FF4 locked you into a bunch of classes and ended up giving you the option of 5-6 at the end of the game in most versions, party swap in the GBA version is a very welcome change imo.
I'll get into this in the anniversary thread in a few days, but it really
bothers me how much FF4 restricted the player in its original versions.