Really what I think it does is force you to focus on what's really important. Either way you're always going to feel that there is much more work to be done than you have time for. Having less time will just let you do the core tasks. Then you can see whether or not the core idea gets traction. If it turns out not to be a successful idea(which is most likely) you'll have wasted a lot less time.

A large number of famous startups have started this way--Facebook, Yahoo, Google, craigslist, eBay...

It is incredibly easy on the vast internet to work on things that may be interesting, different, unique, cool, but in the end don't attract a big enough audience to pay the bills and beyond. That's why I think it's dangerous to go full time until you have some site with significant traffic & growth rates.