Well, in everyday use you will almost never use your 2-hour. It's generally only for planned situations and emergencies. I'll give you a few examples of those though.
Last week I was helping open Banishing Gates in Garlaige Citadel. I (like the horrible n00b that I am) made the dumb mistake of recasting sneak right next to an Over Weapon. It promptly aggrod me and started taking 150 point chunks out of my HP. I trained it back into the tunnel so it wouldn't attack the rest of my party, and did Chainspell > Cure IV > Cure IV > Warp. Instant escape, no deaths.
When you're a much higher level (58+) you will start to become relied on as the parties quick exit. If you've ever seen a Black Mage cast Escape, you know it takes forever. A Red Mage with Chainspell and a Blm sub can cast Escape instantly.
It's also a great way to close out a Chain 5. Say you just finish off Chain 4 and your puller grabs another mob. If you don't think you can make it within the time limit, try Chainspell nuking it to death.
In this case (the Rank 5-1 mission) there are really two ways Chainspell can be used to finish the mission. First, I can use Chainspell to turn me into a nuke machine gun, burning all of my MP to cause fast damage. Second, I can take on the role of tank. As the "escort skeletons" spawn, I grab their attention and then use Chainspell to keep Stoneskin, Blink, and Phalanx up constantly. It's not something you can do for long, but Chainspell + Stoneskin is basically Invincible. Similarly, Chainspell + Blink = Perfect Dodge.
Hope all that nonsense helped.