Factory alarms beep, I have 2 smart passions, one beeps and one doesn't. The one with the factory alarm does the beeping. If you disable the door alarm you'll probably disable the warning when you need it to go off.
You can tell if the doors are properly locked by watching the taillights and blinkers. If they blink THREE times then that's confirmation the doors are properly closed and therefore they will lock. If you do not see THREE blinks, either combination of one or two or three doors and/or hatch glass are not completely shut.
Same goes for any new MB vehicle.