I use a WLF a lot with Medium Format, but with 35mm the ground glass screen is that much smaller so some of the attraction of the WLF is lost for me. However, I have one for my Pentax LX bodies and do use it, either for work with people when the camera is on a tripod and I want to maintain eye contact, or as a lightweight alternative for low level shooting.
I don't use it for copy work - because the Pentax magnifying finder is better for that job, as it is also for low angle work or when angling a big lens upwards. I don't use it as my low angle solution either, if I don't mind the greater weight and bulk of the action finder instead. However, all these cases are only because the LX gives me another finder option that is even better than using the WLF, but if that was all I had (in addition to a pentaprism, that is) then certainly I would use it for those things too.
No, it isn't an essential, but it is a useful thing, and you may find that you really like working with one.
Peter