I agree, I think it would be bad for the team and mess with the balance of the show if they did a couple. I actually think the show had handled it rather well. The few things they did they kept subtle and unrealized (JJ/Reid and Morgan/Garcia got to the point where it might have been at the brink of something, they defused the situation gracefully by bringing on Kevin).

I actually think that the outside relationships can be kind of neat (like all the episodes where both JJ and Hotch have to agnonize over whether or not to tell their families about a certain threat), but it works because we don't see those outside characters like Kevin or Will or Haley in every episode. They are there and occasionally add some depth but they don't take over.