The service supports either programmatic inclusion using the google jsapi library, or linking directly to their hosted copies.
Good things about this
- You save bandwidth costs, as your users download the libraries from google
- Your users get faster browsing ON OTHER DOMAINS, because the more people that use a single source, the more caching should happen
Bad things about this
- Google have more opportunity to don't be evil - the biggest concern for me is their increased ability to understanding which users use your application, and propensity for understanding how they use it. A malicious individual within the company could put some nasty code in the js to screw with your site, or monitor and transmit back to googlebase what the user is up to. It's very unlikely, but it could happen.