Re: weird extra steps: the idea isn't that they're cool. The idea is that if you are willing to attempt it and solve it successfully, it says something about your problem-solving skills. It's not the be-all end-all, but it seems like a decent first-pass filter.
Re: cultural mismatch: if it's a cultural mismatch, you probably shouldn't apply anyway. The thing about a startup is, there are five or ten of you. This isn't just another job. You generally don't just come in at 9, work work work, maybe take lunch with your teammates, and trip it out at 5. You don't just attend the company Christmas party. A startup is typically very much like a family, because everything is riding on everyone. When someone quits IBM, the teammates write it off as a “whatever”. When someone quits at a startup, you spend some serious time looking around to make sure there's nothing scaring them off, because every individual counts a great deal.
In short, culture is critical, and even as a married father of two, signing up for a startup is signing up for a culture and a tight-knit group of friends as much as it is signing up for a job.