Shartley seemed to agree with you on all of this... but I'd like to point some things out about what you stated.

After all, they're just civvies with no training.
Very true, but you're playing a simulation game. It makes you better to pick up on techniques refined through the ages of actual riflemanship.

I'm no expert, so I could be wrong, but remember than not all firearms practices are applicable to paintball marker handling.
Agreed but...

E.g. There is comparatively no recoil in markers
Compare my Tippmann A5 to my 5.56 M4 service rifle. I'd say the M4 has no more kick then the A5.

and the weight is probably much lower too on average.
God this depends on the setup, I've seen some markers here with so many gadgets on them I would be tired lugging it around with my style of play.

Both of these can be seen the photos submitted so far. The fellow standing up has a loose grip in both hands and so does the fellow below with one.
Like you said, this is one of the differences in paintball. He's running a double trigger, as to be expected.

It also depends on fitness. The average civvy will be tired at the 2nd half of the day with maybe an ache here or there. They will show this through how they are handling their marker.
Couldn't agree with you more

To sum it up, It depends also on the day in age your shooting. Preferably in a gun fight with no armor my body would be turned trying to lessen my width. With my SAPI plates in, I will square up on the target preventing a round from entering the side of my vest and by luck hitting my plates and... saving my life.

Remember, tuck the chicken wing, shoulder the rifle, weld the stock, squeeze don't pull.

__________________