I am able to to put Title, Description and Keyword in Products using yoast. I haven't noticed it slowing down any of the sites I created either. Also creates a robot.txt file in a single click, sitemaps with a click, htaccess control and I don't think I would actually want description or keyword generation as I'd rather write rich, unique content/keywords. Honestly I think its just a matter of personal preference more than anything as they both seem to get the job done.
Keyword and description generation are based on the content of the article/product descriptions so are completley relevant and give a great time saving head start. It can also be disabled in the settings. But editing pre-existing content is way faster than writing from scratch.