I don't have hands-on with a D5, but on our old D3, we always use the proper size negative carrier for the negs being printed. In our experience, the 4-blade mask thingie has two uses:(posted 8292 days ago)+ Masking down a large negative if you are only printing a portion - we frequently did this for free-size litho negatives that we just put between two pieces of glass (vs. a standard negative carrier).
+ Preventing light overspill, both around the easel blades (i.e., that could fog other paper waiting to printed or that was just printed when doing multiple prints) and the negative carrier (that could cause uneven brightness or diffraction flare in the image being projected).
It's not the most essential piece of enlarger hardware, but it can be useful...