Spec and class are entirely different concepts. I would never really expect for a druid or a shaman to switch between balance to feral or enhancement to elemental. The gearing is just far too different. In cases where melee or ranged are weak, it wouldn't really be fair to get after a hybrid simply because that's the role that they fill.

However, when a mage says to me, "It's not me, Fire just isn't strong on this encounter." Then why are you playing as Fire? Go Frost or Arcane instead.

The reality of the matter is, no one is really going to pressure a player based upon on single encounter unless you've reached really dire straights on it. But if a player consistently has low DPS on every encounter every week, then it isn't the spec, it's them. And if it is their spec, then why won't they change it?

This is a game, it is meant to be enjoyable, and players should play which spec they wish; and they can do that when it doesn't hinder the rest of the raid. Once content is on farm, sure, they can switch, they can play it while solo or running dungeons or doing PvP, but if being in a certain spec is actually the direct cause of their poor raiding performance, then they have no business playing that spec in a raid.

As per items -- weapons can make a significant difference, but weapon distribution should be averaged throughout your raid. A warrior going from a T11 weapon to a T12 weapon isn't suddenly going to be doing thousands of DPS more than another warrior that's still using a T11 weapon.