Well you don't need to know everything just to use UNIX, but to do things quickly without spending way too much time reading man pages, you need to memorize a lot of things.
My desktop at work is Linux, and I can get by pretty well, although sometimes I find myself spending a lot more time looking at man pages than I'd like to.
The thing is - I've noticed that even though we have things like Eclipse and other GUI tools available, a lot of the UNIX people still like doing all their work on the command line. I painfully watched one girl slowly changing directories, copy & pasting file & directory names together, stumbling through vi... when I could have done the same thing in Eclipse in 5 seconds.
I guess I'll just never understand UNIX people and their fascination with command lines & their hate of GUIs.