Works for me. Like when I have a 6mm mortise chisel...but need to chop a 5/16" wide mortise.."hopscotch" down and back....1" wide chisel to shave the walls smooth...

But, then again...I cut the tenons first...and size the mortise to the tenon...I can always bare a mortise a bit bigger, then trying to fatten up a tenon...

Suppose to allow for a layer of glue? Like the thickness of a dollar bill....