i agree with you on the fact that spybot or ad-aware don't catch everything, but i personally would recommend spybot over ad-aware. first of all, it might use less memory because it isn't skinned, it has TeaTimer which has better registry handling than Ad-Watch(it was a nightmare when i used it), it has an Immunization feature(which i'm not sure works but it's still nice to have it), and i just like it better.
also another thing that a LOT of users don't like to do is scan the comp with anti-spyware or even anti-virus programs because it takes a long time(up to 1 or 2 hours). so just blocking the stuff before it gets to the comp is sooo much better than waiting to scan and then get rid of it.