If it was built in properly, you'd be able to configure which apps are allowed to send notifications etc.

You can do that with Growl right now. Also, users that don't like it don't even have to consider it if it's 3rd party. I see no problem with the current system. Apple has had a million chances to buy Growl and never has, it's not going to happen.

I don't really understand why they would buy Growl. It wouldn't take long for them to add in to OSX themselves.

Many third-party applications use Growl for notifications. Buying Growl and maintaining the API would mean that Mac OS X Margay's exciting new Notification Centre feature would have instant support across a wide number of apps.

The actual cost of buying Growl is negligible to Apple.

It's Apple. All they have to do is issue a beta and they'll have plenty of people writing to the new API.

The cost of buying Growl isn't just the dollars used to take ownership of the company. You have to integrate the people and the code. For a feature as small as this, it's just plain easier to implement internally.