Except if you go to the ground to wrestle one person, you are vulnerable to anyone else around you, there's also the gun that you have on your hip. Once the suspect is on the ground, the fight should be mostly over, its a matter of putting restraints on a person who is prone at that point. BJJ works well on the ground against a single opponent, that does not mean you want to be on your back with an armbar when you are arresting someone in a parking lot, at night, with five of their friends standing next to you. Yes, you will stack the deck and get backup if you can, but if the altercation begins before you have called for the backup from dispatch, now you have to do that, protect your weapon and arrest the guy who you are fighting with. BJJ is used to fight back to your feet or as a last ditch defense if things go to the ground. If you make a habit of trying to subdue suspects by wrestling with them on the ground instead of taking them down and then getting on top of them you will absolutely get hurt on the job or worse.