As stated above. Empty magazines would make ammo for them extremely rare, and quite a commodity. In turn, it will allow you to spawn more (now empty) mags, and make certain weapons (CR527) much more viable. It will also allow you to make weapons (sometimes, rarely) spawn with mags attached, because they will not be immediately available to use, and thus, not OP.
It is a non-issue with small mag weapons, like CR527. But, if you consider the huge mags (double stanag, akm drum, upcoming saiga drum, upcoming saw), they would then contain a ridiculous quantity of ammo - more than any player would need in a lifetime, basically.