I'd wager that the rear half of the van would detach before those hitch bolts sheer. The extra little carriage bolt held to one thin section of unibody almost seems silly. So, I didn't bother with it. These vans ain't no tow pigs. I've got the Cummins if I need a four-wheeled tugboat. I have two small enclosed trailers, and I will probably utilize them with the Sprinter from time to time.
First.
Of course all of the included fasteners provided by Curt should be installed.
After looking at the position of those bolts I agree that they likely provide little extra benefit for Class II towing. I believe that Curt claims Class III rating. Those bolts might be necessary for a Class III load, but the NAS aka NAFTA 2500 Sprinter isn't rated for over 5000# dead weight anyway.
CURT 13265 Class 3 Trailer Hitch (T1N 2500 Sprinter)
"DEPENDABLE STRENGTH. This trailer hitch is rated for 6, 000 lbs. gross trailer weight, 600 lbs. tongue weight, 10, 000 lbs. weight distribution capacity and 1, 000 lbs. weight distribution tongue weight (limited to lowest-rated towing component)."
Use with a weight distribution setup would change the stresses. For WD I would install the bolts. BUT... MB never mentions or recommends the use of any weight distribution type towing that I have found.
Any "normal" aka dead tongue weight will attempt to pivot those 90 degree angle pads to press up against the frame, not pull away from it.
The Draw-Tite 75210 design Sprinter hitch doesn't have a similar 90 degree pad. They do use 4 thru bolts on the frame though.
Added:
A compromise might be to install a heavy self drilling screw and washer in the back section of the tab hole. A self drilling screw provides good shear strength.
vic