That makes a lot of sense as a motivation for writing OFCs, and indeed I've written some for that reason myself. It definitely applies to the Silmarillion also -- there are more female characters than in LOTR, but fans who want to write more than one woman together still often need to invent OCs for that.

Plus writing in that world from a perspective other than the heroes is always fun.

I agree! And I think Middle-earth is a great world for OCs.

I'm glad it was interesting to you!