The worry with an all-volunteer army is that the people who are in it WANT to be in it, and perhaps there is a mindset with that wherein it's easy to 'just follow orders', right or wrong. So maybe C-in-C says, "We are declaring martial law in 12 hours. Saddle up. Anyone balks, shoot 'em." It is possible that a drafted infantryman/woman would refuse. A volunteer? Maybe not.

I have though about that argument for years. I do NOT know how much water it holds.