Of course Flt Sim is no substitute for real flying. But it is useful! One of the early problems students have is where to look in the cockpit and how often should look in. Knowing were and what to look at, and indeed how long to assimilate the required data is vital. For example, one needs to know one's speed on finals, but 95% of the time one should be looking outside. By not getting fixated with speed, one will achieve a better mix.
Later, during applied flying (especially instrument flying) Flt Sim becomes even more useful.
But you cannot learn to fly by using Flt Sim!