Why? It avoids potential intra-group arguments - better that a group of like-minded people have the style of game they want, rather than watering it down to accommodate one other player who would be better suited to another group.
We played a 4 hour session on Friday evening, it contained 3 fights, which lasted a total of 4 rounds (less than 10 minutes in total), the rest of the time was roleplaying, planning and exploring... the group did everything they could to avoid direct conflict where possible. Afterwards they all told me it was the best session of D&D they'd ever played. All 4 players were fully engaged in the plot, and due to playing characters with weaknesses, they were constantly on edge, aware of their frailties and the danger they were in.
Add an optimised character played by a powergamer whose aim was to dominate battle and the whole dynamic of the session would be changed - lost - broken.
We do not subscribe to the character 'roles' put forward by 4E, so if someone came to our table and started asking about tanking, defense, control etc, or started referring to a character as a 'build', they simply would not fit in.