It baffles me how anyone can consider Latin 'useless knowledge'. I know of no other language which opens so many doors to so many European languages, and most certainly to English as a second one.
I had some Latin in school, and yes, from time to time it comes to mind when encountering a new French or Italian word. But even if you want to learn several Romance languages, I'd guess that just learning them and recognizing/exploiting relationships is easier than also learning Latin. It does add a common denominator, but it also adds a lot of cruft.