I tend to agree with the opinion of, what appears to be, the majority of older members posting in this thread. And that is, in order for the class system to make any sense, then there needs to be something to differentiate them. If you can have whatever weapon you want, then we may as well just have team death match, and be done with it. Personally, I don't think that would be as much fun.
The classes force you, as a player, to make choices. You can't have it all, so you need to decide what's more important for the map at hand. Are there speeders or vehicles that you plan to use on this map? Then maybe you want to be a pilot for this map. Or, if the other team has vehicles and you don't, then maybe you need to be a class with a rocket launcher. Is this map focused mainly on close-quaters combat? Then you'll want to be a soldier. Is this map without vehicles, but combat at long range? Then perhaps a sniper is in order.
Don't you see what would happen if people could pick up whatever weapons they wanted? People would either kill themselves, or have their buddy kill them, then respawn as a different class, and pick up their dropped weapons. The result is that everyone would pretty much be the same, and that's what's not fun.
All the world's a stage, and all the men and women merely players...