Not that the possible solo areas should disappear, far from it... To each it's own I'd say. Sometimes I like to solo too.
Or where you need a rogue to advance. Like a trap which only he or she can disarm. Or where you need to have a very high search skill to find a hidden trapdoor to get further down the dungeon.
Or another where you have to sacrifice a Paladin to advance... err... strike the last idea
But I think you get the gist...