No reason NOT to have several magazines for any firearm that uses detachable mags. When I got my first AR10, mags were somewhat difficult to find and relatively expense. Lately, large frame AR mags are both available and lower priced. Since the intended purpose of my 6.5 Cr and .308 AR's is hunting, most of my mags are 10 rounds but on the slim chance something more is needed, I've been adding a 25 round now and then. That said, adding a 25 round mag to a lightweight platform somewhat defeats the weight advantages.
I recently purchased a bolt action Ruger American in 6.5 Grendel. The determining factor was the use of AR style magazines which I already have on hand.