So I only have 20 points to spread across Soldier, Medic, Operative, and Engi?
No matter which class you choose to be at any moment, you'll always have access to the basic necessities for the class - the upgrades make you better at the class you choose to upgrade though.
A lot of players are planning to kit themselves out as only one class. A few of us (myself included) think upgrading 2 classes will be better - MIGHT have moments where we have to switch to non-preferred classes, but we should be able to find some utility on at least one of our chosen classes, whereas a single-spec Medic who's in a game with 3 other single-spec Medics will be a little short on options.
Also, dual-spec characters won't lose as much effectiveness switching between their classes as a single-spec character will when switching from their chosen class to anything else.
You don't HAVE to spread your ability points across all the classes - remember you're part of a team, work with the people you're paired with, and you'll be fine.