From the wiki:
Current Last Name
This field could be called Preferred Last Name, Last Name at Death or Married Name.
For living people, it should be whatever they would currently prefer.
For non-living people, it should be the last name they were using at the time of their death. If a woman had multiple married names you can include the others in the Other Last Names field.
I don't really see the point of it.
1, Typically used to store married name, but this can already be derived from their marriage relationship so I dont see the point of duplicating the data.
2. Of course if they are married more than once then it doesnt really work, you now have to add the first marriage name to other last names, once again cant this just be derived form their marriage relationships.
3. For living people its the name they currently prefer, therefore its related to the date it was added, its transient, is only definently correct the instance it was added. This is contrast with most information which is correct when added and will remain correct.
4. It has multiple meanings, for example what do you enter if someone is married but they prefer another name, i.e you cannot rely on it meaning anything in particular.