Large problem:
People spec a lot
People tend to quit games a lot of the time
This means you will inevitably waste time convincing specs to join
This pisses people off.
1 person missing in a 11v10 is not badly detrimental, especially if missing person is n00b and doesn't want to change.
These are setbacks
Positive
More balanced
Makes people more inclined to join.
Stops idiot team creating e.g 4v5