I understand the benefit of having a character whose skills are all completely focused into one class, but I can also see the benefit of a player who can play more than one class. Being able to play two classes, or even just having one main class and a secondary class could be just as much of a help as the super-specific spec'd players. A soldier who can switch over to a medic if your team is getting hit hard might be a big help at times, even if you just add enough skills for the basic revive and heal. One thing that SD has said multiple times is that they are making the game such that someone new to the game is just as useful (with respect to skills and weapons) as high-level players. Which means that a medic, or any other class, with only the basic skills is still useful.
Its also possible that players may prefer different builds and classes for different maps, having completely different characters for each map. Even if its as simple as one character for defense and another for offense, its not hard to believe that someone would play multiple classes equally.
I'm not saying i expect to see many truly generalist characters, able to play any of the classes equally, but expecting to only play one class in Brink (let alone in each game) is really limiting yourself.