Chatting with him isn't going to change the fact that I'll usually value my pick more than his. If I can get Soraka or Janna before the enemy, I'm going to do it. I'm not picking his stuff if I can get my stuff instead.
Besides, if I'm third pick (the earliest place I could have a ban), I'm not in a good position to actually pick his stuff for him anyway. I'm still going to be picking after 2-3 of their players. That's too many opportunities for the enemy to pick whatever OP bullshit that I don't want to play against. I'm perfectly happy making sure no one gets it.
And finally, you're assuming that I can get him to chat with me and actually effectively tell me which of my champions he has in a timely manner, which is a major assumption in and of itself. I have rarely been able to trade with someone without having to pull teeth asking which supports or AD carries they own.