I used to do this when I didn't have any viable DD job at 50, for fun. As in, having 2 healers in the party already and going in a 8-man instance as a 3rd healer designated to DPS 100% of the time. Sure, Summoner is great, but not when you have 30 MND and are not that geared to begin with. That being said, the content I've done this for has been pretty trivial when all is said and done. Titan HM, Garuda EX - stuff of that ilk. It's pretty fun but underwhelming in the end, because if you optimize your play (mainly on a SCH), you'll have the time to off-DPS while doing your healing duties as well as you are required to do. More engaging!

I'm the most boring person ever so I haven't really partaken in any of those "6 scholars, 2 tanks" -like festivities, but that would be fun. Pretty sure I saw a video of that for Titan EX and Ifrit EX. Hmm, might be a good idea while waiting for 2.2 to do something silly but fun like that!