Okay. It’s very simple. I don’t know why people don’t get it.
Choice of abilities is a tactical decision which you make as you level your character. If you plan on running solo all the time, and you find that there are rarely enough skilled players on your team to justify specializing, then don’t. Pick up a few abilities for each class and then you’re set. It’s not like that’s not an option, and based on what we’ve seen so far you don’t need every ability for you to have important stuff—with 20 abilities, that’s enough to have 4 in each class and then 4 general abilities as well.
If you find that you often will have a specialized player of every class, then feel free to select one or two classes for your character(s) and specialize. Then, if you absolutely have to switch classes to one that nobody has specialized in, then your tactical guess was WRONG and you lose because of it, just like if you pick rock when your opponent picks paper.
For some reason, if you are playing rock-paper-scissors and you pick wrong (despite having no knowledge of what your opponent will pick, just like here) it’s obvious to people that you’re stuck with a bad decision, but in Brink where rock DOESN’T always beat scissors they feel like they should be able to choose paper after the fact anyways.