What’s the benefit? It sounds as though it would lock a shell plate in to place, but why?
Just picked up a new Pro 1000, but in researching it before buying, I'm guessing the pin is there to make sure the shell plate is properly aligned.
One of the problems that the older version seems to have is having primers getting seated sideways. Among the theories on why this happens is the timing on the shell plate is off so the primer pocket is not aligned with the primer. When you go to seat the thing, the primer will catch the edge of the pocket and get flipped. With a physical pin there the plate should always be aligned correctly.
FWIW, I'm also using a 30+ year old 3 hole Lee turret press, and the timing on it, which mechanically is a lot like the Pro 1000's setup, is pretty hopeless where I line the thing up by hand after it attempts to do it by itself (,even after changing out the plastic piece that's supposed to control it.) Since I use that press for the more precise target loads with a Hornady Auto Charger it's not a an issue since I'm usually waiting for the Auto Charger to finish trickling the powder.
Another thing I noticed, the pin sticking up through the hole in the plate gives a reference on how far down the plate has gone, and a visual cue to know if the primer has been fully seated or not. Probably not that important once you get a feel for the thing, but it did help with the first few rounds I put through it.
There seem to be a lot of people who like to beat up the Pro 1000, and I'll admit some of it is deserved. However, if you're reasonably intelligent, attentive, and understand what's happening when you're working the lever, the thing works.