My 2 cents.
Bear in mind that I have never been in the military, or in a gunfight, hope I never am...
First, I own an AR-15, I absolutely love it. My favorite gun to shoot. I probably have 1500 rounds through it so far. It has been FLAWLESS!!! Yes, I have cleaned it fastidiously, but I do so with every gun I own, whether I fire 500 rounds or 10.
Second, our military is the most advanced and dedicated in the world. Doesn't it make sense that if there was a "fundamental flaw" with the weapon system the military would adopt a new one? Body armor is one thing, bad rifles are another entirely.
Take the M9 for example, it has its critics and its supporters. I personally think the Sig p226 would have been a better choice for our troops. (ask the SEAL's)
Regardless of that fact though, it has a good track record, good service length, and has seen us through many conflicts. Also, woudn't you know it, the military just ordered a whole lot more from Beretta! Must work!
The Military continues to order the M4 platform, and continues to use it to great effect. I understand the criticism's, and maybe some of them are valid, but to me, the proof is in the pudding, and the AR/M16/M4 have a track record of success and longevity in our armed forces.
Feel free to flame me, but I have my 2 cents in.