Because I built my 1975 Kawasaki Z1-B 900 (1100) from parts (I started with a bare from from a seller in Oz, then bought close numbered engine cases from a seller in the US, wheels from Oz and the US, forks from the US, etc etc) I initially bought a lot of repro stuff, which I'm "backing up" with good used OEM parts date stamped (all Z1 major components were date stamped) to suit my 1975 build.
I have a good used tank, sidecovers, ducktail, front and rear fenders, tank and sidecover badges on their way to Oz from Nils Menten in the US, so with all the other OEM parts I already have, the only parts I probably won't bother with is an OEM exhaust, for either of the Kawasakis. I have no need to ever sell them or put them back to stock, so when I turn up my toes, my sons can either keep riding them, or sell them, and the new owners can pony up the dough for repro pipes if they're gonna go down the restoration path, to me they're just bikes that I want to ride. Of course if I find some OEM pipes going cheap, I may jump on them, and hang them up with the other stuff.