You can't advertise on them, no. You should probably try Blogger instead, or just get your own domain and get Wordpress.org on there.
You can promote affiliate products or sell links probably but those are still better done on your own domain actually
Yes, you can make a qucik buck using wordpress.com blogs if your blog is good W.T.O Traffic and ranking . Selling Links can give you a lot of revenue !
I would say just go with a blogger blog if you want a free blog, as it's more well-known as opposed to Wordpress's free blog host. However, hosting a blog on your own domain is always the best option of the 3.