I pretty much concur with this guy's general point.
The universe is very big, so...
1. I'd bet life was relatively common
2. intelligent life very rare
3. Advanced, spacefaring intelligent civilizations rarer still
4. Chances of one being nearby and having "visited" us highly - even astronomically - unlikely.
Let's say for example that there are a large number - hundreds of thousands - of technical civilizations in our galaxy alone. A hugely optimistic estimate. If they're distributed more or less evenly, the closest one to us might be a couple of hundred light years away. Optimistically. But even if there were one that close, there are hundreds of thousands of stars in just a 200-light year radius around our Sun. Our radio signals haven't had time to go out very far (we've only had radio for a century, and the signal degrades proportionately the further it goes out into space), so to a hypothetical advanced alien culture sitting that close, all of those hundred thousand stars might be equally worthy of exploration. From their perspective, they'd notice nothing particularly special about our system. Why would they come here, especially? It's the ultimate needle in a haystack, and asking a blind guy to find it, even if he knows what he's looking for.
Bottom line? I don't think for a minute that "UFOs" are alien spacecraft.