I use the combo locks that are the 5/8 pin to retain the drawbar.
I feel they definitely stop the impulse grab and go theft. Sure if someone has time and will power they could defeat it...
BTW I've heard of pranksters taking the pin so your trailer comes off the truck somewhere down the road...they help prevent that too.
I used this kind when I was RVing. I agree with Diamondpilot that if a determined thief wants to defeat it, he will, but it'll take him a little time and discourage amateurs. The lock also made sure the hitch stayed latched in transit.
It would be hard for a thief to use a hacksaw to cut them off, but they might with a power saw.
The way my Reese hitches are. It would be very hard to cut off the lock for the receiver, because there isn't no room to use a saw.. But I guess a thief could pack around O/A and cutting torches.
I put 3-in-1 oil on mine and I've never had a problem getting mine off.
When I was looking for trailer locks I came across a YouTube video of a guy picking a Master trailer lock. It took less than a minute, I think. No hacksaw or O/A needed and helpful tutors on the web