Comparing Seven to Twenty-one or Seventy-Two doesn't make sense. As you said Seven works as a name. So if parents happen to love a number that works as a name, like Seven, what is wrong with that? Nothing. It works fine so if it has special meaning to them they should use it.
It's the same with tree names, if you'd like to name your child after your favorite tree and it is a Willow
, go for it. If it is a Bottletree, a Dodo Tree or a Prickly-ash, then don't.
What I mean is that if they happen to love the number 7 and the sound is kind of name-y I don't see a problem with them using it.
I can see why many people don't like it and that's fine. I just don't think a child would feel horrible about being named Seven. Of course he could, but he could be just as unhappy with a name like John
. You just never know which name your child would like to have.