The only thing you can't learn on a desktop flight simulator is the sphincter-clenching fear you experience the first time your plane stalls (loses lift and falls from the sky.) When a Cessna stalls, it drops approximate 500 feet before it's moving fast enough to gain lift again. If you were coming in for landing and below 500 ft, a stall (which happens when you're airspeed is low* such as when you're... I don't know... landing) will kill you.
The experience worked though. After I solo'd I almost stalled my plane on approach but realized it soon enough (the controls get mushy) and guess what... I lived.
Incidentally, the other thing the desktop simulator doesn't do is allow you to experience the sheer joy of flying a goddam airplane! I never got my license (lost my job and never went back to it) but if the FAA allowed desktop simulator hours to count towards training, I would still choose to pony-up the money and
fly.