That said, it could always be a prompted option when you have the follower join your party for the first time ("Would you like to sync your follower to your level?").
Something like this would have my vote - mostly because I'm obsessive-compulsive and like to have all my party members with the same EXP value, but also because (as Gyson mentioned) the difference between a level 1 and a level 2 character, for example, is so noticeable.
