No need to comment Just a straw Poll.
Uhm, have you ever tried doing that? Because you'll find really quickly that it doesn't work. You either end up with a bunch of lonewolves or poorly used assets. Roleplaying has nothing to do with it.
You need separate squads because:
You need to communicate to the rest of the team what assets are available to them. If your mortar squad is called "Harambe" and is full of Infantry players running around, how is another squad able to know that they have mortar support at their disposal?
Separate lines of communication are important for the team to effectively use assets. If you're driving around in an APC while your squad mates are operating mortars and doing infantry duty, how the hell is the rest of the team going to communicate and coordinate with the mortar component of your squad? Are you going to be an intermediate while you're engaging an enemy squad with your BTR?
Separate chain of command is necessary to ensure that the assets are available to the team and that they're used effectively. Any asset incorporated into a squad that has a different primary function will eventually only serve that squad's primary function. Infantry squad with mortars? Mortars will prioritize whatever target your infantry component is engaged with. You will frequently ignore other squads' mortar support requests because it would affect its ability to support you. And so on, and so on.
Asset- and function-based squads are essential to any strategic and tactical complexity beyond infantry squads haphazardly crashing into each other.