"Instant Preview" has to be the single worst change that google ever made to their search pages. I'm at the verge of switching to bing for a while because I keep triggering these god-awful popups by accident.
Bing has hover previews too, they just activate with a gesture to the right rather than a mouseover. The content of Google's previews is far superior though.
You might be right, though I have a feeling I might be part of one of those "1% experiments" that Google is known for. Right now I have Instant Search off, and it also keeps thumbnails for showing up for search results. However, I turned Instant back on, and the thumbnails still don't show up (at least not automatically). I guess YMMV... sorry for the misleading info.