You have to wait 10+ minutes for transactions to be _irrevocably_ confirmed, but with PayPal the relevant number is more like 3 months. Most practical situations where you need instant transactions can probably get by with zero confirmations, although the issues around this aren't quite trivial. If zero-confirmation transactions _do_ turn out to be too risky in practice for widespread use, there are other ways of working around this without entrusting the whole payment process with a trusted off-chain third-party.
The problem with this is that anonymous transactions (eg a bet) can be double spent without any adverse action on the sender, because he is anonymous.
Zero confirms work great if you're sending Bitcoins to your friend or buying a cup of coffee (this isn't taking in account tx / dust fees through), but if you are running a web facing app that allows anonymous usage.. you're going to go bankrupt if you accept 0 confirms on the blockchain.