If I'm on the phone with a client and ask them to look at something I can generally figure out what browser they have (minus the version) by asking 1 or 2 questions and that's often good enough.

Now... if you used this data in a way to automate some typical developer->client BS then I could see paying for it maybe.

For example, let's say you load up a few VMs with every major browser / browser+version combo.

Then you can tell a client to goto some url, maybe it could be something like:

http://browser.somedomain.com?for=somepage.com/preview

If the client goes to that address it captures his browser details just like your site does but then:

1. Replicates his settings with one of the VM browsers.

2. Takes a screenshot of what somepage.com/preview looks like.

3. Sends the screenshot and data to my e-mail or have an API letting me get that data back in on my end.

That has pretty high value because when a client says "the border looks weird!" that means nothing to me. I have to see the problem to be able to fix it and people who aren't technical are horrible at explaining technical problems.