DD can't cure themselves.

Do you really think, in a party situation where debuff actually matters (boss or NM, not grind parties), that Conjurers will be spending MP on doing combos for a possible Debuff instead of curing that party?

That would have to be one heck of a debuff. Curing is very important. Unless all the mobs can be like Ifrit and DD never get hit, conjurers will be curing and buffing with stoneskin exclusively.