Simple and effective SEO. Non-invasive, elegant. Ideal for client facing projects.
The usage is simpler as the keywords field is disabled by default. And the WordPress excerpts can be turned of for descriptions.
Yes you can. Check out 'Add keywords field to post editing screen' in plugin options.
If you are doing some custom stuff (for example) with your templates, it might come handy. We need it.
It's all there.