Both Group Queues are for those who have a group to go and know who they want to play with. Both DM Queues are there for those who know, 'I want to be in a DM match'.
Consider the new 4 queue options: Group DM, Solo DM, Group Random, Solo Random. Solo players can do both so are more likely to accidently go with the default. Clearly groups need to select group to group. A new solo player getting brutalised when they face a coordinated group is not a good thing for the health of the game so making solo the default makes sense for this reason alone. More importantly, encouraging new blood/new players. Its not 'lowest common denominator' its 'providing maximum convenience for maximum amount of players'.
