Small rooms, foggy swamps, twisty tunnels... there are lots of places where such a party would be suboptimal. Keeping both ranged and melee capability around is definitely very important to any party, just because the counters to all-melee and all-ranged are so readily available. (For example, our party is now hitting the third encounter in Storm Tower, and it's the first one where all-ranged wouldn't have been pretty heavily screwed... and even this one is a smallish room where we're certainly not more than a move+charge away from anything, even corner-to-corner.)