With the possible exception of groupon and dropbox, all of those names would very expensive to acquire. The problem is that a startup typically has to pick a name before they have $50000+ to drop on a domain.
Mint.com on the other hand was purchased with ~$180k in Series A stock. The domain owner ended up making $1+ million when the company was acquired. The founder (Aaron Patzer) seems to think it was one of the very best decisions he made.
The paradox of choosing a domain is that you buy it when your company is small but you live with it when your company is big.
It's a lot like choosing real-estate for a restaurant. You can launch your restaurant in a bad location, build it up, and then move. Or you can find a great location and build it there. A great location doesn't mean you will be successful (it's just a big advantage), and a bad location doesn't mean you will fail (it's just a big disadvantage).
thefacebook.com didn't, to pick one example.
Sure if you have a couple of millions in the bank, that helps but I'm more thrifty and always feel that it would probably not change much to find a different name with an available (or cheaper) domain.
[1] http://gigaom.com/2009/11/24/dropbox-raises-7-25m-crosses-3m...
We'll likely write up a blog post in the near future disclosing this along with some additional information.