You can only run so many A/B tests at the same time, as mentioned earlier - 1 to 3 priorities a week. So any tool that enables you to focus those A/B tests on real pain points that will deliver significant improvements when fixed is extremely useful. That's just what UserTesting.com does, for about $30-40 per test. They pay people to go through your site, perform tasks that you set, and speak out their thoughts as they do so.
As David Alison puts it:
There is something incredible about watching a complete stranger go through your web site, vocalizing what they are finding as they go through it. Though I have been through hundreds of usability tests over the last 20 years, that didn't stop me from saying out loud "Just click the Features link!!!". Seeing them trip on tasks you thought would be obvious or immediately understand something you thought would be complex is very powerful as a site designer.
Seems like an essential tool in the startupper's toolbox.