I feel like I remember reading an official response on this, but I can't seem to find it atm.

As for exploits, changing classes during a loot roll might be able to cause some problems, but if it's a vote probably not too many. They might as well put the same limiters as dismiss/abandon (minus the minimum wait time) to be safe though.

I've also seen a few of the opinion that it will cause many dungeon runs to "require" people (read: many people expect of others) to have multiple geared jobs at 50. For instance, bring 4 dps (SMN+BLMx3) or something else for all the trash in a dungeon and then have one person switch to tank/heal for bosses.

I myself am not of this opinion however, I think changing class inside of duties would be a good addition. I am of the opinion "why not?"

I'm sure some other people can think of problems, but I really hate having to consider trolling and whatnot as reasons to not add a system.