This is no big deal if you only cannibalise your own fleet, but once you start coveting widgets belonging to your Significant Other you are living dangerously.
The situation became critical recently when I took the Zeus seatpost off Donna's road bike as part of the ongoing pimp-my-Hurlow program.
There was nothing wrong with the original clamp that came on the Ideale No.90 alliage legere saddle, but a recent saddle swap on my Holdsworth Whirlwind had liberated a set of Zeus-for-Ideale saddle clamps that cost too much to leave unused.
These also fit Campagnolo Nuovo Record posts, but, nah, it wouldn't be the same.
Luckily, ebay provided a NOS Campagnolo Gran Sport post for Donna's bike before my stuff got put out on the front porch.