[kristofferR]

Well, you could, but that would leave your wallet (containing all your BitCoins) at a remote server, removing much of the point of Bitcoin. It would be just another Paypal basically (although you would be able to send the money between different "Bitcoin-PayPals" without a fee).

It would also make it much harder to send Bitcoins to other devices/people. With a native app you could for example detect other Bitcoin clients in near proximity via BT/same Wifi to make it much easier to transfer money, with the web you're stuck typing a long cryptic address.

[cookiecaper]

Check out MyBitcoin. Something like this is actually very useful. The wallet on the remote server can (and should) just be a temporary transfer; send to mybtc or a similar service on sale, transfer out to a private wallet once a day, etc. Then, when you want to buy something, mail to your mybtc account, and then to the vendor you want to buy from. Only one extra step and still much more convenient than typing in a btc address.