I actually have been using my Uncle Mike's pocket holster for this
www.gunaccessories.com/UncleMikes/InsideThePocketHolsters.aspI don't wear tight pants, so my reach into the left front pocket with magazine extraction is quite smooth and fast.
1) I do not have to "clear clothing" such as shirt tails or jackets. I just slip my hand into the pocket.
2) I do not have to raise my shoulder and arm to get my hand on my belted mag, THEN, raise my arm even more (or contract the arm more; whichever is more like how you draw) to withdraw the mag from the belt carrier. I just drop my hand down to my pocket and come up with a magazine in the correct orientation for reload or whatever.
3) I do not have to worry about anyone seeing my mag/mag holder or just the bulge.
4) I do not have my mag/carrier catching on doorways/countertops/etc.
5) I do not have to worry about someone "touching" my magazine during a hug or casual body contact.
I just make sure not to carry anything else in my left pocket. It is exclusively my ammo pocket.
If I were down on the ground, I can see where it might be more difficult to reach.
But then, a belted mag might be hard to get also, just depends on the circumstances.
Another plus is having one less bulky accessory to remove from my belt if I need to remove everything to be locked up before going inside a "no-gun" facility.
I keep a lockbox in my vehicle and I can dump my gun and mag into the box and secure it.
That leaves one leather holster (a VMII... yeah, you guys got to me finally!) to put away.