Are there advantages to having more than one private key per wallet? Suppose I was to design a bitcoin client that supports the ability to have multiple wallets (i.e. multiple users).

Is it safe to assume that each wallet is identified by one private key?

Is there a disadvantage (from a privacy/anonymity aspect) to having only one private key per wallet?

Are there any advantages to having more than one private key per wallet? If so, what are they?

EDIT

I was under the impression that a private key can generate many many public keys, that's why I asked the question. Given that a private key will only generate one public key, the question becomes meaningless and it becomes obvious that each account should have lots of private keys (without limit).
Well, you don't really want one private key per wallet, you want one private key per transaction (and this is what the default client does anyway).

But you may want to have more than one wallet, because otherwise you might be mixing "accounts" that you want to keep separate: If a single transaction output is not enough for a given payment, the default client will use multiple, and that way your otherwise unrelated "accounts" become connected.