To be fair, it's a sound argument. You'd have to do some kind of search like {completely_nude -accessory -hat} (although accessory isn't currently populated) to find "completely nude" characters, which goes beyond the normal user limit.
I don't see an issue with hairstyles being limited, since as Obstetrics said, "I can claim usefulness to me given I enjoy the "natural beauty" aesthetic; something unhindered by jewellery and tied hair.".
I don't think tagging it is an issue either, in case a mistag happens another user can correct it.
You could also make an argument that some pixiv tags are prone to being misused by people who misunderstand what they mean/don't particularly care about the tags' meaning.
Then again, having 3 separate nude tags would be bothersome, and functionally speaking birthday_suit would just be a more restricted version of completely_nude. Maybe a pool could be better?