That's going to happen for a while until people start to settle into the idea that "soldier" is a specialty in Brink. It was probably a poor class name considering how specialized the other names sound and how interchangeable all the classes are as actual combatants.
I'm starting to consider Double-Class Utility Players (I'm going to coin the acronym now as D-CUPs) a bit more as we learn more about abilities; it seems more likely that a character will be able to fully develop and still specialize in two classes without giving up much over a single class Specialist.
Which begs the question, what combos might be the most beneficial? I'm thinking an Op main with a Medic secondary could be quite useful, and maybe Medics doubling with Engineers.
If the information in the videos remains accurate, there 9 abilities that can be chosen per class (and used in addition to the 1 or 2 abilities that default with that class).
Level 1-4 characters must choose Universal abilities. This leaves 16 class-specific abilities that can be chosen. Therefore, you can completely spec out one class with 9 abilities and a second can be almost completely spec'd out with 7 abilities (or 6 if you want to throw in another higher-level Universal ability like Silent Running).
Still, assuming my guesses are relatively close to the reality that soon shall be revealed, we should be able to have pretty competent dual-spec'd characters.
I personally favor these combinations:
Engineer + Medic
Medic + Assault
Operative + Engineer
I assume you meant Soldier with the Assault.
Medics and Soldiers will always be useful, since they keep the team alive and healthy and supplied with ammo.
Engineers and Operatives definitely have less direct but far more complex combat support roles. Engineers will be valued for the weapon buffs and CP upgrades, while Operatives get lots of cool toys to screw with Engineers' defenses (and Engineers can screw them right back with destroying the hackpad as seen in the Get SMART Objectives video).
It's therefore natural for this pairing of classes to dual-spec in each other (Wraith and I independantly planned to do Engineer/Operative builds with opposing primary emphasis). So the Soldier/Medic and the Engineer/Operative makes sense.
I could see how the Medic/Engineer could work as a team-buffer, that would essentially be the http://www.gamesas.com/index.php?/topic/1181826-master-support-build/page__view__findpost__p__17516681.
Operative... yeah, that doesn't really compliment the other classes well, given the radically different playstyle (everyone else is more about helping the team, the Operative is all about disrupting that).
Soldier/Engineer would be the Master of Blow [censored] Up, Have a Nice Day.