A new-found ability of ret paladins in 3.0 is a little something the community likes to call crowd control. Repentance, the 31-point ret talent, got a bit more than a spit shine when the devs were polishing the talent trees for Wrath's release.
Previously, Repentance was only usable against humanoid enemies, and was a fairly short 4-second incapacitate with a minute-long cooldown. With all those restrictions, I saw it as a PvP tool to interrupt casts at range and slow someone down in order to close the distance. Unless you were in a humanoid-rich instance environment, it wasn't even a valid ability in PvE. Bosses are immune to it as well, of course.
Now, however, Repentance has an expanded eligible target list and more uses. It still has a 1-minute cooldown, but against PvE enemies, it lasts for a full minute duration (unless broken by damage), and can be used against demons, dragonkin, giants, humanoids, and undead. Go over that list and think about it - what do you encounter besides those? Beasts, yes... but not much else. It's instant cast, can be used at range (20 yards), can be used in combat (unlike a rogue's Sap ability), and can be chained against a target, since the cooldown matches the duration.
So when your instance group is having trouble with massive pulls, thin the ranks out a bit - pick something the tank is targeting specifically, repent it, and have the mob of enemies moved away from the repented target so it avoids being hit with splash damage. If you keep an eye on the repented target, either via focus targeting or just splitting your attention, it shouldn't bother anybody.
Want a way to do it all with one button? Well, I can offer a combo of buttons - here's the mage "Polymorph" macro, adapted for use with Repentance:
#showtooltip [target=focus,harm]  RepentanceTo use, here's what you do - target something, hit the macro. It'll repent it and put it on focus. Any subsequent presses will attempt to repent the focused target (but it won't be eligible for another minute, since that's the length of Repentance's cooldown). If you want to repent something else while your original target is alive, target the new baddie, hold shift and slam the macro. Voila! The new baddie is repent'ed and now considered your focus. Easy, no?
/cast [target=focus] Repentance
And now you know why everyone loves running instances with mages. Well, aside from the food and drink conjuring...