Let alone requiring a round trip for the server to even see the information... does that mean a 302 to itself for initial data? What about a POST? The first render is where this kind of crap is most useful for server-side rendering.
The fact is, in general, it would be nice to get the actual browser/version and os/version, and maybe have an engines list for cross-compatibility detection as a fallback.
When you come across a bug that only effects a specific browser/version (or other browsers using that engine as a base), it's a pain to deal with... cleaning it up when fixed is another issue, or worse, when you target a specific version, then the next release doesn't fix the issue and you're scrambling to update your range for the fix.
It isn't nearly as bad as the late 90's, but it's still pretty bad.