There is a valid reason for this. Take the new IAR for example. They don't accept PMags and in the heat of battle, it's inevitable one will be fed into an IAR and that precious few seconds can make all the difference.
Mags are disposable. GI mags are extremely reliable if people accept the fact they're disposable, just like PMags. Throw them out when they reach the end of their life cycle.
PMags are great, don't get me wrong, but there's not much advantage over a new GI mag. If you read through the thread I linked below, pay close attention to who's posting. Failure2Stop is a Bronze Star recipient. A couple guys are special operations. Iraq Guns is a contractor who services hundreds of rifles regularly. You'll see many prefer GI mags, though some prefer PMags. There's no "better", it comes down to personal experience. I can tell you that I've seen a higher percentage of broken PMags than I have GI mags. Anything but black PMags are more prone to failure also, which I noticed a number of years ago. It must be the additive in the compound, I'm not a chemist (if that's even who would know!) so I don't know.
The fact is, the GI mag is a great magazine. Throw them out when they reach the end of their life cycle. Same with PMags. Try not to run them over with a truck then they won't bend or break as often. There are many magazine related failures from PMags, they're not what many make them out to be. Keep in mind as I say this, I have more than a few dozen loaded and ready to go, and I trust the one's I own 100%. I trust my GI mags 100% as well. If a mag fails me, I paint mark it and it goes in the training mag pile or the garbage, depending.
Personally, I think it's a good idea to stick to just the GI mags. They work in every rifle in service while the PMags do not.
Big Army says no more polymer mags - M4Carbine.net Forums