Sorry, as an academic librarian by profession, who spends much of their time helping students search the huge numbers of databases available these days, I see many of the name and other fields available on WikiTree as different ways to add 'keywords' to assist people to find a profile and not create a duplicate.
This is particularly important with European Aristocrats who can be known by a range of names and/or titles.
So I completely agree that the Last Name at Birth (LNAB) and Current Last Name (CLN) should be what the person used, but the Other Last Names (OLN) is the field to enter names that they may not have used themselves, but are known by that name later. Particularly as it is one of the few name fields where multiple names can be entered.
I agree with Isabelle that both Montmorency and Montmorency-Laval are appropriate name to add to OLN to try to avoid duplicates being created who have come across that name in Wikipedia.