You got it with the shim, it just provides additional entrances for foraging bees to reduce congestion. This also allows the bees quicker access to where they would store, and not have to progress up thru the brood chamber & an excluder.
I however just used auger holes on the supers to provide these additional entrances. Some prefer not to put holes in the supers, but I find it easier than having yet another piece of equipment to manage as well as deal with brace comb built in the additional space. However when used properly, the claim is brace comb is not an issue.
The only draw back that I have found is when manipulating, and as you get down into the brood area, you have field bees returning looking for an entrance that no longer is there. Whereas, with just a bottom entrance you never have the additional comotion of field bees looking for the entrance. I find it a minor draw back.
As far as robbing and wax moths. For a strong colony, there will be plenty of bees to guard the minimal entrance area increase. If your hive is not strong enough to fend off ants and wax moths, they ain't gonna be strong enough to store excess honey. In fact, on my non-DE modified hive (no ventilation box) I elevate the outer cover as well which also increases the entrance area that needs to be defended. Robbing is not an issue when there is a honey flow, or at least I have never had a problem with it.
The excess supers also give room for the bees to spread out more and reduce their temperature. With the extra supers, I find less bearding in the hot weather.