a paiement label is not necessary.
Sure it is "not necessary" but still would be useful for people.
Perhaps some people would like to keep one BTC address for some reasons.
Also bigger financial institutions won't really like the idea of different account number for each transation.
I don't think the keys should really be thought of as an account number per transaction. That logic I think is misleading. You can have millions of keys associated to one wallet / account.