While I always enjoy the bleating about the supposed superiority of the M1E6 rifle (often erroneously referred to as the M14-why a failed P. I. P. justified a new designation is beyond me) every time I refer to the direct-impingement AR as near perfect (gas pistons are anathema to the faithful!), some people may not know the reason. The AR, as designed, uses a small volume of propellant gas to push on a key attached to the bolt carrier, driving the carrier back against the buffer spring to cycle the action. Most other self cycling rifles use a bit of gas to shove a piston against an operating road to push the bolt. ARs can be readily modified, adding a piston and op rod, adding parts and making the system more complex. In what way could this be useful?

First, it is theoretically cleaner. The propellant gas used to drive the action is blown into the magazine/chamber area, and contains carbon, commonly called fouling. A gas piston conversion traps the fouling ahead of the piston, meaning the cleaning process adds a step. The chamber area should always be cleaned on any rifle, with a piston the gas trap area must be cleaned as well.
Second, it is theoretically cooler. Without hot gas being blown into the chamber area, the chamber doesn't heat-saturate as quickly. This means that the rifle can have more rounds run through it, faster, before cooking off.

Note that both of these advantages are theoretical. It take hundreds of rounds (thousands, in some cases) to foul a D. I. AR enough to stop it running, especially if it's lubricated well. If you refuse to clean a rifle, please don't shoot it. It pains me to see good machinery abused. If you have a select-fire AR, too much money, and little sense, it is possible to run it hard enough to overheat. It takes about 10 mag dumps, with fast reloads, to do it, though. If you can afford to dump 300 rounds, in less that a minute, without hitting anything smaller than this planet, don't do it-take a new shooter out teach 'em.

So, if you keep your AR clean (please!), and don't use it as a S. A. W. (it's not designed for it), you don't need a gas piston. If you still want one, go ahead, but mere personal preference does not equate design superiority.