The CSD is a very useful member of crew if they choose to be and, in fact, when they do choose to be they could quite easily replace another crew member in the service.
They could easily replace a crew member and be part of the service. I also think some passengers would appreciate it. I had a CSD once coming back to WT asking to do a trolley because he missed it! There you go. It can be done.
On the 747 and 767 they could work in CW and on the 777 in FC (or CW depending on config) because of the location of their office. It would also give them a good variation of cabins to work in.