Would This Houserule Bother You: Unusual Backgrounds attached to high ability scores

[Rendu]

It's a house rule- the only people whose opinions matter are your players.

That being said, Unusual Background is just a catch-all term for exactly this kind of situation- where a player wants something that is outside of the "normal" limits or expectations of that particular campaign. They're asking for some leeway, and paying Unusual Background is the price for that leeway. You don't need any more justification than that.