Hmmm, I don't know. While it's true that Quake and Merton also hurt your party, they differ from Purifier as they can be absorbed. Purifier causes non-elemental damage, so if your party has low HP, you could potentially loss the battle if you summon Crusader. It has been pointed out by a few people (including myself), that Merton can be used to both damage the enemy and heal your characters, but it would have made far more sense for it to teach you Ultima, the best spell, which you receive much earlier in the game, learn at the same rate, and you don't have to kill any optional bosses. The MP bonus ( which i did originally note), is rather substantial, but is almost useless later in the game, as you can easily get econimisers (well, sort of. Fighting brachiosaurs is always a pain) for all you magic users.

As for the Omega Badge, it was a piece of crap, but after beating the hardest boss in the game, what other items do you need? There are many more bosses more powerful than the 8 dragons (the rest of the bosses in Kefka's tower, Magimaster, Wrexsoul etc.) so I think it would be more appropriate to compare them to Shinryu. Shinryu was also a "super boss", yet when defeated, he gave you Ragnarok: the most powerful weapon in the game.

I can see what you mean with meteor, and I'm not saying that Crusader is completely useless, but I would think that the hardest Esper to aquire in FF6 would be the most useful. Espers like Ragnarok (Ultima), raiden (+2 vigor bonus and Quick), Bahumut (+50% HP is much better than +50% MP, plus you can learn flare) and tritock (+2 magic) I consider much better, and are much easier to aquire.

If Crusader could cause 9999 damage to the enemy or taught you Ultima ( I always take ragnarok as a sword), I would understand. Crusaders abilities make it a good Esper, but it's not the best.