They're not. Depending on your competency, you have a _ton_ of tools at your disposal for filtering traffic ranging from basic throttle to sophisticated behavior/request profiling.
I've spent more than a little bit of my career dealing with bots and I'm not really sure that a cryptographically signed blob proving that the request came from $thisSpecificVersion of firefox running on $thisExactVersion of osx is really going to help me.
I don't care _what_ made the request because that can always be spoofed; this cat and mouse game always ends at the analog loop hole. I care about what the request(s) are trying to do and that is something that I figure out with just the data I have server side.