Pilots who teach themselves on a simulator can learn how to fly with fewer crashes, but as many pilots discover when their skills plateau after only a few short years, just because a person can fly, it does not mean that he or she is flying correctly.
Most people think that getting better at making corrections and stick time are the main requirements for better flying, so little thought is given to how they fly, or whether they are flying correctly. Most pilots therefore make 4 to 5 times more control inputs than what the maneuvers require when flown correctly. As a consequence, self taught pilots typically experience slower rates of learning, inconsistent landings, and difficulties adjusting to new models and wind.
For these reasons, whenever possible you should seek guidance from a competent instructor who can explain and monitor proper control usage, rather than attempting to learn by trial-and-error and merely reacting to what the plane does.
Simulators can be terrific tools for practicing new maneuvers and building confidence, and are therefore highly recommended. However, developing good habits and thus ensuring long term success requires instruction.
See also: DAS System (Knowing How To Fly)