I have recently acquired a dCS NB used - and I am pleased I did!
But getting ahead of myself here - even though I’m not using a dCS DAC (yet) is it worthwhile adding an external clock to the NB? Maybe not because both the DAC and the NB should sync with the master clock, but I thought I would ask anyway.
I think you answered your own question. The critical point from a clocking perspective is the point of Digital to Analog conversion. Synchronising the clocking of whatever is feeding the DAC with the DAC itself ensures the clocks don’t drift apart which avoids dropouts or pops, but adding a master clock to the NB alone probably makes very little sense.
Once you have a dCS DAC you will be able to use the clock in that DAC as the Master clock for the NB. If you like the improvement that makes then adding an external Master Clock should give you even more of the same.
Just to add a little to the very good answer from @struts001 . There is no guarantee that a random external clock is going to be better than what Network Bridge already has. As dCS have written on numerous occasions, it is not just the clock’s accuracy that is important but its stability. IMO many people are buying clocks on the basis that they now have an extra box and that must be good mustn’t it ? 
Having spent the money any change to the sound is held to be an improvement (even if timing may be worse if measured).
Thank you - that is an important point. If the dCS NB already has an excellent clock (a stable clock in this case) I must tread carefully when considering other external clocks that are not necessarily a significant (if any) improvement.