I think this is a rehash of the "pigtail v. no pigtail" argument of years ago.

Ask him how he would remove the receptacle for service (on a temporary basis, of course), but keep the "downstream" circuit live, without cutting, stripping, and joining the conductors.

(With the pigtail method, of course, you could simply cap the exposed ends and blank the box - until you return with the correct color receptacle [Linked Image])

BTW, the actual wording for multiwires is

NEC 300-13 ('96 version - it's what I have at the FD) Mechanical and Electrical Continuity - Conductors

(b) device removal

In multiwire branch circuits, the continuity of a grounded conductor shall not be dependent upon device connections, such as lampholders, receptacles, etc., where the removal of such devices would interrupt the continuity.

I'd just use the argument that a good practice should not be limited to a single application, just because it's not required.