Well, let me see.
No worries about the wind lever poking your eye out.
The rewind crank won't bend or break or get caught in your sweater like on M4s and up.
The film reminder dial on the back can't be set wrong (unless it's a IIIg.)
No pesky rear door flipping open when loading film.
Absolutely no RF patch flare.
No irritating framelines or focus patches cluttering up the view except on that oddball IIIg again.
No frameline preview mechanism to get stuck, requiring a cla.
Lighter weight than Ms, so less likely your neck strap will break, and it's easier on the strap lugs.
Just try setting 1/40th second on your M. When you need it, you've got it.
Fewer lens options to empty your pocket.
Screw mount lenses allow more time to savor the lens swap process.
Longer effective RF baselength than all but the M3 and uncommon 0.85x models. A full 1.5x magnification, with a variable diopter to boot.
Brighter viewfinder.
The frame counter is much more readily visible than on any M, even the M2.
Trimming the film leader keeps things in perspective that your Barnack is unique and special.
The vast majority of Barnacks were made in Wetzlar, not Canada.
Uses the same cable release as the Nikon F and F2, saving you at least $3.
Sturdier 3/8" tripod thread.
T.