Blu is talking about the Bnet and file structure in general, and Blu is usually right about most things.
However, the Blues from Tech Support did comment on this specifically for D4 on the Battlenet forums.
I am with Blu though, if someone has room feel free to put them on a backup drive or keep them around. It is not likely they are useful, but if you have space and such, I don’t see any harm in leaving them.