Yes, playing a game like this months in advance could definitely influence someone's opinion.Yeah, this attitude bugs me a lot. It STARTS with the assumption that everything is crap - so anyone suggesting that something might NOT be crap is simply blinded by circumstance.
You also strongly implied that negative impressions are more valid than positive ones, because positive ones can be explained away by the special circumstances in which conventiongoers find themselves.
That said, going into a match with the opinion that the game will be terrible (based on what has been shown) would influence an opinion just as much.
(Just to be clear: I'm not saying I want the multiplayer to be bad at all, if I was giving off that impression.)