<div style='font: 9pt ; text-align: left; '>The problem is you need mana to cast all those spells.
The point of a 2 hour is to save your life. Before 40, 95% of the time when the situation goes sour, you're already at half mana or below. That doesn't buy you much time.
And casting a ton of heals, nukes, or enfeebles/crowd control spells is going to get you major, major aggro.
After 40, this leads to the next problem: Using convert when you do have aggro (or a very high risk of it) is you signing your death warrant. Which makes the whole "I won't run out of mana, I'll chainspell!" factor a moot point. You can't exactly spam a ton of spells when you get your head taken off with a single hit.
Paladin using Invincible is about the only time when you can spam the spells with Chainspell and get away with it.
Ultimately, Chainspell is a situational tool. That situation is running like hell.
Level 34 RDM/BLM: Chainspell/warp. Get out of trouble in 4 seconds flat, assuming you can survive the pounding during the warp animation.
Level 58 RDM/BLM: Chainspell/escape. Get your party out of the dungeon in an instant, without the 15 second escape cast time.
Level 72 RDM/WHM: Chainspell/Teleport. Same deal.
Aside from those, Chainspell is pretty much just a novelty. I used to use it when I knew I'd level on that battle, so I could spam a bunch of nukes. I got hit, but it was nice to use it for something.
It's not as useless as, say, the Ninja 2 hour, but the simple realities of aggro management make Chainspell iffy at best.</div>
"For one thing to live, another thing must be killed. God, or whoever is responsible, deserves respect for creating such a system. It reveals that we cannot take all matters into our own hands. We need only to focus on doing our best--a simple, yet profound way to live, is it not?"
