Sounds the same as how I remember the system in WoW to be but still, lets try the system before we claim its broken, shall we.

There are ways this kind of exploit can be avoided. For example, the exp a mob gives is based on the highest level character to attack the mob (similar to FFXI if they were inside the party, not sure if FFXIV is the same). Then, in this case, since the high level character will have such a large difference in level with the mob the parties exp will be low to non-existent.