Not on my bike but in my head. (Please refrain from any loose cog jokes as I have heard them all before).
I keep thinking about all the differing opinions on the parting out of bikes. I know of one reason I do it that has not yet been mentioned. I learn a lot about the various parts, how they work, how they are disassembled and how they are re-assembled. It is sort of home schooling if you please. I don't have time to sit around my LBS all day to learn and besides they already have work to do without me getting in the way. I don't live by a master builder who could teach me the ropes but I do possess a better than average mechanical ability. I also have the curiosity to pull something apart to learn more about it.
Some of you here take that knowledge for granted because you are so close to it every day. Some have even become (dare I say) snobs about it. Some forget the days when they first learned that the left crank is left hand threaded. Sounds simple but watch a newbee try to take pedals off and you will have a sense of what I mean. I have learned so much by trial and error plus asking people like you questions. This forum has been a gold mine for that. I can still remember one of my early restorations where I installed an NOS Simplex TDF rear derailleur on a bike and it looked great. I posted a photo on the web of the bike and had several people (some from this list) point out that I had the chain threaded backwards. I was almost arrogant about it because it looked fine, like every other derailleur the way the chain rolled across the pulleys. How was I to know that the actual routing was counter intuitive or what looks like backwards. When one of you sent me a catalog image of the correct installation I immediately corrected it. It worked fine the other way but still it was wrong.
So next time you wonder why someone would part out a bike or keep a pile of parts at the ready for a project think of what it was like when you picked up your first wrench and the shop owner said "no do it this way"
Cheers and Happy New Year to you all