I think it's a perfectly reasonable houserule. I see no reason to deny something to melee types that casters can get, in relation to wearing heavier armor. If it's balalnced for a Beguiler or Bard to be able to cast with no chance of ASF in medium armor (or a Duskblade in heavy armor), I can see no harm in allowing a Ranger to use all of his abilities in medium.

In a recent thread, I managed to track down a suit of special full plate in the Arms & Equipment Guide (Tessellated Armor, p.95) that counts as medium. If the DM allowed you to add mithril to it, it would count as light.