Andrew, the difference with buying normal ads (as opposed to links) is that ads don't affect search engines, while buying links that pass PageRank does affect search engines--and all search engines, for that matter. Imagine if you bought an ad in a magazine and you started to get better "rankings" from the magazine in terms of more positive coverage. Most people wouldn't want that.

To your last question, Google absolutely does have hundreds of signals that we use in our ranking; we don't rely only on links. It's a hard problem to assess quality accurately, but we try to find different ways to do it.