Sure, there are browser plugins to override the HTTP user agent header and in my experience impersonating Google Bot works quite well. On the other hand, this tactic runs afoul with sites detecting your UA for browser specific hacks (especially with HTML5 video) or parts of a sites where bots are blocked by policy.

Also the IP ranges of search bots are more or less well known, so it would be trivial for publishers to block this kind of hack, if they wanted.