Is there any reason NOT to carry a grandfathered mag?
Since the only alternative to my just-barely-evil 16 rounders is a ten rounder... yes. Interesting that our fifteen round limit started out in debate as a ten round limit and was amended. But it nonetheless often has the effect of a ten round limit because manufacturers often don't make a fifteen round alternative. (If I carried a Glock instead of a CZ I might be better off in that regard.)
Even when I carry my compact, with a 14 round mag, the spare mag is the same 16 rounder I carry with the full size. The gun came with a second 14 rounder, but I leave it empty for faster logistics at the range (I swap the ammo back and forth every few months to give the springs a rest--plus I download by one anyway). I sometimes think about ordering another fourteen rounder to use as spare. Then I realize that no one has supplied me with a compelling argument against carrying a grandfathered mag.