Strong-personality-assholes have been making successful things for a very long time now (Linus, Steve Jobs, Larry Ellison, come to mind).
It's probably not that being an asshole is good. It's that if you are a genius/visionary and nice, your genius gets diluted by the people around you. Let's build a genius/asshole map:
1. Genius, non-asshole: You start a genius-level thing, it is brilliant and awesome. As it progresses, less brilliant people get involved. They start wanting power and influence. You know that the things they want to do kind of suck, but you're too nice to tell them that. Over time, they ascend to positions of power, because you, out of niceness, allow them to, and your thing descends into mediocrity.
2. Non-genius, asshole: You're just mean to everyone. If you start any sort of thing at all, it goes nowhere, because it's not very good, and nobody wants to work with you.
3. Non-genius, non-asshole: You're nice, but so are lots of people. This goes nowhere.
4. Genius, asshole: You start a brilliant thing, because you're brilliant. You're mean to people who deliver anything but the absolute best results. So who stays with you? People who always deliver the best results, or, alternatively, people so enamored with your genius or the thing you've created that they're willing to tolerate it. Congratulations, you've managed to filter your underlings to only the best and most passionate people - you have a high probability of achieving something great.
Note that this is not an endorsement of this behavior. Just an attempt at explaining why it is that these types of personalities seem so common among highly successful people.