The gil issue aside I think it is worth doing merely to give people some alternate way to level that doesnt involve FATEs or the limited # of leve's. MMO's are about options, I believe having sidequests as a 1 off seems to be a huge waste in development cost and time for one 3-5 minute piece of content.

Other MMO's have you level multiple characters so the sidequests are done repeatedly, now I am not suggesting unlimited retries as that would be abused but a reset now and then maybe once every expansion or when a new class/job is added might not be a bad idea