I had a VFG on my WASR for a few years when I had it set up with a T-6 stock and railed foredends and a red dot (before I got my Bushmaster M$ and put it back to the "traditional" AK setup). I found it VERY comfortable for me, especially with the grip as close to the magwell as I could get it (which isnt as close as you can do ut with an AR due to the heavily curved AK mags, and the way they have to be inserted VS the AR). Not that a semi AK has much recoil at all, but I did find I was more accurate, more comfortable, and able to get faster follow up shots with it. I am only 5'4" with somewhat short arms though, so for people with longer arms,YMMV a lot.
Didnt bother putting one on the M4, as a main goal for me was to keep it as light and handy as possible, and, as was mentioned, the magwell works just fine, and is very comfortable for me, so it just isnt needed.I have tried shootign the m4 by holding the fore end "normally" and it's fine, being that the gun is so short and light, but I was still a little better and more comfortable using the mag well.
As far as using the sling to steady, I'm sure that is probly more stable,and is liekly great for the range, plinking, hunting (depending) when you have time to slip into it,but as mentioned, I can see where it would make doing a lot of maneuvering that way (like in house clearing in Iraq) awkward, or getting the gun from a "down" position, onto target, the sling would slow you down. So, my non-expert, non-combat experianced opinion on that is that I havent seen any video of Marines using the sling on thier M4/M16's that way, and sticking to the VFG or magwell and I'll asume thats why.
Could be wrong, I was (am still actually) just a Squid that fixed electronics for aircraft weapons systems, so I have NO combat/military gun training AT ALL (they handed me an M16 with a "laser tag" type deal on it in boot camp and said to fire 10 rounds at the target prone, sitting, standing kneeling, with no instruction at all, and the scores didnt count at all, for anything, and you couldnt fail
) Guess they figured I wouldnt need a gun, since if you got past all the ships whose #1 purpose was to protect my carrier,and then our the F/A-18's, F-14's, CIWS, an M16 wasnt going to save me.