In its announcement of this feature, Square-Enix went out of its way to emphasize that it has built in the system safeguards against abuse by powerlevelers and gold farmers. There are also some reasonable limitations intended to keep the game streamlined, fun, and balanced. For example, equipment-based stat bonuses will scale down with each level a player drops.
There are explicit details at the official Final Fantasy XI website about how experience gains and equipment scale. We approve of this addition (as if you needed us to); in fact, we wonder why certain other games still don't have it. Being able to play with friends regardless of their time commitment and experience is a big positive, and we can't even think of a substantial drawback.