So everyone else is writing rants and overly long opinions in their blogs, I might as well be one of the cool kids too.
Well, I don't see why people who are very logical most of the time are forever bashing Cait Sith's battle mechanics. (Or his role in the storyline, which was an odd combo of Quina and Auron, but was both hilarious and greatly important, and I need not go into if you were paying attention to Reeve half the time he was on-screen. Let's go over what's so useful about Cait Sith:
- Materia slots: The Silver M-Phone is a very early obtained weapon that has a Slot value of 8. IIRC he has the first 8-Slot Weapon of the permanent characters (I'm not factoring Aerith or Sephiroth into my rant for frankly obvious reasons.) Right after you get the Edincoat which can be equipped by anyone and has a Slot value of 8 (IIRC, again) bringing him up to a total of 16 Materia Slots just after beating Palmer. This is highly relevant for some other reasons we'll see soon.
- Materia growth: Again, excellent early-on for a permanent character. His White M-Phone and Black M-Phone don't exactly have a lot of materia slots, but are both obtained around Cosmo Canyon and give 2x to Growth. That's the best until you get Cid's Scimitar and Cloud's Apocalypse, but those are obtained very late game.
- HP: He has innately high HP (although I think Cid's and Seph's might be better) and with all those materia slots, you'll probably have some slots left over, right? So add an HP Plus or two and watch as he becomes the ultimate tank!
- Enemy Skills: Normally magic contracts HP, but Magic Materia is actually pretty weak in this game. So the best spells come from Enemy Skill Materia, which has no effect on HP. E-Skills also are long range (just like magic) and typically ignore Reflect. Therefore, you can stick him in the back row, with incredible HP, and he has all the things great about, say, Rydia of FFIV but without the horrid defenses. So like a cross between Rydia and Kain. Tack on Cure and he becomes an excellent tank.
- Limit Breaks: Oh goodness yes. And you don't have to waste time training them since Slots is learnt after 40 KOs. That is again between Cosmo Canyon and Palmer most of the time.
- Dice: Not too great. You can get lucky and deal ~2400 damage on Level 8 (the lowest possible level to recruit him) but this is easily Cait's worst aspect.
- Slots: I never did like Setzer or Wakka much, but THIS is great. Get good at it and you can reel up Mog Dance -- which functions like Pulse of Life without the Life2 side effect; Toy Soldier -- like a powerful Slash-All; Lucky Girl -- which has a 50% chance of DOUBLING the damage of all allies or reverting to the weak Toy Box... you see, the possibilites are endless. And of course, Game Over insta-kills all enemies, even Ruby or Emerald, but it only has a 7% chance of going through.
- Long-lastingness: Most of these factors last only until Disc 3 when the others catch up, but by that time you've got Long Range and HP Shout to compensate for his weaknesses and still keep up his tankiness. Besides, by that point, everyone is just a clone of everyone else with a couple flashy tricks (Barret's Ungarmax Lucky Sevens, for example) so why bash? I'm convinced Cloud/Cait/Yuffie is the best final team by far, and Cloud/Yuffie is well accepted (mostly because of Yuffie's Conformer + Clear Tranquil resulting in overkillage and invincibility), but Cait is a great complement to the physical aspects of the other two, so why not?