Its very useful to be able to assign a unique ID to every fixture. This ID travels with it wherever it goes, and only disappears if the fixture is deleted from the file / rig altogether, so it is a cross check when it looks like something is out of order etc. 
This is also invaluable if the show is to be transferred to another theatre, tour, re-mounted some years later etc.
Capture must anyway assign an internal unique ID for everything in its database; maybe you could just allow in the case of fixtures for this ID to be optionally exposed (or at least a truncated version of it since its probably a string of gobbledy-gook that Computers like , but Humans find difficult to read)?

In any case ideally the computer would automatically generate the number whenever a fixture is introduced (which must be what happens anyway?)

cheers

rabbit