Growl is great for all the reasons you state, but Chrome doesn't use it because Growl doesn't allow HTML, and it would require us to ship Chrome with Growl for all the users who don't have it.

HTML notifications can be powerful because they allow you to include live content (video of the person attempting to video chat with you) and actions (e.g. archive, reply buttons) if you choose.

It's not inconceivable that someone could write something that suppressed and stripped the text out of a Chrome notification and presented it in a Growl notification.