I restrict races, too, but my reasons are different-slash-arbitrary:
* "Monster races", such as dragonborn/tiefling/halforc/etc, are restricted to one instance per party to ensure the feel of rarity (this is narratively expressed in the game, such as in settlements). Yes, even the PH optional races (half-elf excepted). My game world is human-centric and I don't want a party of drow, kenku, and aarakokra messing up that vibe. But in the interest of flexibility and compromise with players I permit one monster race in the party at any given time; I'm satisfied with this and my group hasn't complained (and FWIW current characters are all human).