Difficulty isn't the point. It's an MMO, which is a type of game people tend to enjoy playing with other people, particularly their friends. If there is a system in place which makes it more difficult to and/or penalizes you for playing with your friends, it's not a good system, and deserves to be looked at for tweaking or removal. There are millions of people playing various MMOs all around the world, but sometimes you want to play with a specific handful of those people (these are generally called friends), and you should be allowed to do that without anything hindering your enjoyment of the game or their company.

That said, level syncing would be a much better way to facilitate friends having fun together than simply widening the existing rank gap.