I'll give you my last flight search (and ticket purchase) as a direct counter-example.
I want to go to Europe for two to three months. I'll fly wherever is cheapest and I'm willing to go for any period between March and July.
This is theoretically one search on Adioso. It is impossible on Kayak or any other website I've seen to date. And when I say impossible, I don't mean impossible in a single search, I mean I would have to do several thousand searches to cover the full matrix of options which would actually be impossible.
Not to mention, the carriers listed by Kayak actually was quite limited (not sure about others).