It has to be good practice. If you spend 10 years practicing a bad swing its not going to help much.
I agree, perfect practice makes perfect, but practice will still make you a lot better even if it isn't fundamentally sound.
I learned to play golf without any assistance of any kind. I didn't have any family or close friends that played. My only influence was books, the Golf Channel and the millions of balls that I hit. I shot my first round of even par, during a tournament, 8 years after I bought my 1st set of clubs. My swing may be full of faults but I can play a hell of a lot better than without practice at all.
Also, it's not like you can just go get a lesson and all of a sudden be good at golf. It takes a ton of hard work...or fun, if you love it like I do.