^ It's all about variety. In the first game you were simply limited by the space and size of the grids in your inventory, not by certification or class. It was fun not being locked into a class like you will be now, I enjoyed running around in Standard Exo-suit with a Jackhammer. Infiltrators for example could not pick up anything larger than a pistol for their weapon, because it was only a 3x3 grid.
Agile, for example, had two pistol slots and a rifle slot. They could carry whatever weapon they wanted. The old system was about variety and customizing your loadout to your liking, which I completely prefer over the new class based system where you're locked into what you can use.
I'd say keep the classes, but still give yourself an inventory window in which you can customize your ammo capacity/tools/grenades etc, but each class still has a special ability which they can not switch out or use on other classes. It works for both ways. i.e Medics are the only ones that can heal/revive etc, but you can still customize your inventory in a more detailed way than you can in the loadout screen.