i also think that the nature of Adventure Paths is also part of the issue.

i've been playing for over a decade and never had a problem like this. then i started playing an AP that my DM would not customize to us (it's her first campaign and i agree that she shouldn't change things unless she knows what she's doing) and my party was really weak to start (1 class that comes on-line at level 3 and 3 classes that don't until level 5).

tactics became incredibly important while i fought alongside two newbies and someone who had barely played since the old AD&D days. i tried feeding them small suggestions (like fighting defensively and aiding another), which they would inevitably forget about as we got our asses handed to us. i'd sit there silently as they wasted one of their 3 precious level 1 spells to take down a goblin that was running away while their commander was kicking my butt. it was incredibly frustrating fighting these battles designed for four 'average' characters as 4 crappy ones, knowing that if i were DMing I'd have designed more reasonable encounters for the group.

i also suddenly, for the first time in all of my time playing RPGs, became obsessive about loot. Knowing that any missed loot would make us even weaker and would not be replaced by the DM made every missed piece of loot a kick in the balls. parleying with an enemy became a bad choice because the chance of dying was worth the loot that we'd miss and would never be replaced from a wealth-by-level perspective.

anyway, i realize that i got on a bit of a tangent here but it does relate to what you were saying. Player A might be trying to keep up with the AP while Player B doesn't give a fuck. While it's easy to say that Player A is in the wrong, which he is, Player B is playing in an AP and the group might not have the luxury of babysitting him.