Startups have some dreadful lows, so I can understand not wanting to hire people who will leave at the first painful moment, especially because one departure can trigger a fatal exodus. There are also people who just collapse during painful periods and a startup can't afford to have them... but the ones who collapse tend to have a certain passivity about them that makes them incapable of being voluntary job hoppers.

However, we're talking about entirely different sets of motivations when we compare the "stay-or-go" decision at a big company vs. at a startup. Someone who leaves a mediocre corporate job after 6 months because he isn't learning anything and because it's obvious that no one with any power cares about him is, in my opinion, savvy and ambitious. This sort of person could easily be a great startup employee-- knows what he wants (and is therefore less likely to job-hop out of ignorance and confusion) and seasoned enough to cut through bullshit and avoid the wasting of time.