This topic of STANAG vs AUG mags comes up from time to time. And sure, if you really want all the same style mags, then getting the STANAG stock supports that. Personally - I don't agree with that path, but hey - it's OK to like stuff I don't. Here's why I don't in general I prefer the factor AUG mags.
-The AUG magazine system is just better. It's a continuous curve magazine with a very thick-heavy feed lip, that can take a beating, and feeds smooth - unlike the change-directions STANAG for the straight AR magwell. They look better too, but that's a personal preference.
-The AUG is designed to run on the AUG mag. Compromises are made for STANAG mags. Those stocks only have one trigger transfer bar, and don't have the bolt lock/bolt release button (it'll lock, but no button).
-The AUG Factory 42 is the highest capacity factory 5.56 issue magazine of any gun anywhere that I know of, and they're solid dead-nuts reliable mags. Stanag compatible fat-body expanded mags are either too wide to actually work in a Bullpup, unreliable, or both.
-All STANAG mags are designed to work in the AR15 platform. Not all of them are necessarily designed to work in the AUG usage of it. Threads showing weird problems like strange bolt carrier impact wear, or other issues - happen with STANAG mag systems.
And finally - the cost of AUG magazines are not high, nor do you really need dozens of mags like everyone acts like now. 3 30's, 2 42's, and you have a lifetime supply of mags for that rifle. There is no 3-gun stage or any other usage without break that's going to exceed that. More is better, they're cheap - buy more, but you don't need $400 worth of mags to roll.
But in the end, it's OK to like stuff I don't - so if you want a STANAG mag stock - go for it.