how do you mean the tyre sim doesn't affect ai?
just because you read it on those forums? as if the ai always have tyre sim on (as these are your settings you are choosing, not theirs) then you aren't going to notice any difference in their times when you turn it on, as its only affecting you by making it the same as them by taking away your advantage. if it makes you place properly then why not turn it on, it works, its harder with it on, you need to warm the tyres up first and you need to watch out when you go off the track and collect debris.
I really am not sure why people keep saying tyre sim is faulty apart from having read it by some yank moaners on the CM forum.