The reason people feel inclined to just rush through dungeons is because they don't offer anything of worth beyond tomes. On top of that, the dungeons themselves tend to not be challenging, or on release end up being at a level where the majority of players doing them are already overgeared for it. The trash mobs themselves don't really pose a great challenge or really force players to approach them differently. Group them up, AOE them down, done. Rinse and repeat.

As for the Primals, I believe both are canon. Extremes are what the Primals become when you use more crystals to summon them (plus a sacrifice of the person summoning it, I believe). Or alternatively you could say the Extremes are how the Primals really are but for the purposes of DF they have to make the fights easier, hence nerf the level of their damage/skills in the MSQ.