It's not how it affects him now though. It's later on. I have seen it first hand in my brother (now 18 but played these games since he was about 12/13), and it's just an underlying lack of...well, horror at violence. He's still a gentle person in the sense that he would not copy the violence, but the years of playing these games means he is no longer shocked by anything. He watches violent programmes on the television and laughs, because he thinks that is the appropriate response. His understanding of good and bad is also skewed. He can rattle off the names of WW1&2 weapons without thought, but can't explain why the battles were fought, or why we consider the Nazis to be evil, beyond the most simplistic reasons. getting him to understand the difference between hapless footsoldiers and truly evil military types is hard. To him they are all just enemies on a screen.
My DS is a long way from this, but I will certainly not be backing down, and if I suspect that he is simply playing these games at friends houses I will seriously consider a ban on visiting them (though they will be welcome to visit us). As I said, it is not the short term I am worried about - of course they don't copy the violence! But in the long term these children grow up without appropriate responses to violence, and that is not a good thing at all.
And that's quite apart from the addiction factor, which is just as bad. My brother used to get up at 5am sometimes to play these games, or would still be playing well after midnight (obviously not the same day). He lost all interest in other activities, and when we visited my dad would jst sit in his room on his XBox and only leave it for meals and bathroom breaks. It's just not healthy, and now that he's finally developing other interests again, he's got to overcome a weight problem and fitness issues.