Are there any mining pools that allow a specified amount of bitcoins to be sent to a chosen address? This would be like using a pool as a wallet service, as you could send a subset of your balance to a retailer to purchase something and avoid running the Bitcoin client app. 

Most pools only pay out the complete balance to your specified address, which makes this option unavailable.

Obviously, whether or not it's a good idea depends on how trustworthy the pool operator is, and the sum of BTC at risk.
You can use an online wallet to handle all of your Bitcoins.

The reason you don't want such an option in a pool, is that in case someone steals your password, they can drain your account. This is the reason most pools have a 24h waiting period between when you change your receiving address and being able to send any Bitcoins.

In the future someone could potentially make a joined pool and wallet service that will allow you to manage your earned Bitcoins easier, but that is not yet an option (I'm slowly working on it though;) ).