Yeah, that line of thinking can be bogus. I ended up staying at a startup for quite a while, and occasionally had to defend myself for not bailing out even after a couple years. Saying, "Stock options" and winking usually quieted down the critics. There are reasons (even beyond stock options, LOL) for staying at a job for more than just a little while.
BTW a lot of what others have said here (pay raises came from getting new jobs, employers saw long stints at a company as a potential warning sign that you aren't really very good) does ring true.