99% of the traffic has the correct user-agent which is useful in tracking down issues that are browser specific. The other 1% will just get ignored as noise. And it's not like we can't tell what type of browser they are using with browser specific objects that we can pull from javascript. We just can't infer the version of the browser which is critical for debugging issues.
I use a randomized user agent and every once in a while get rejected by a site that has decided my browser can't be supported. Usually when I'm on some Safari version. I doubt much effort is being done to identify fake user agents since there are better fruit to pick in the fingerprinting game.
