I'm with Mosaic.
It would eliminate a whole annoying math process (figuring out XP for those who multiclassed without their favored class)
It would improve flexibility and player choice.
It is eliminating a rule, not adding one. So just one small step towards making the game easier to learn. One less thing to explain to new players.
Totally backwards compatible.
If concerned that it upsets the balance by de facto removing an advantage of humans, just give the humans a cookie to rebalance it. A few bonus hit points or something.
