I really don't see anything wrong with a martial buffer beign able to pick a secondary role; unless picking the Soldier specialty makes you better at combat than a fighter or choosing Scout makes you a better rogue.
For example: a full-BAB class gets the same sneak attack progression as a rogue in addition to other unique features
This would be problematic, unbalanced etc..
It's not a matter of "how many roles can this class cover" imho, it's about not stepping over other classes areas of expertise.