I've got two bikes that I would consider collectible, a late 70's Mondia Super and a mid 80's Merckx Professional.
The Mondia is more or less equipped "as I found it" - that is, it was a bike that had obviously been raced hard and wrecked a few times. Consequently, I doubt it was "original" when I found it, and so I had no qualms about having it repainted/restored to a more pleasing aesthetic state. During the rebuild I replaced damaged parts with period correct replacements, and had the original Campy hubs laced to new rims. Keeping in mind that even "original" condition Mondia's do not garner a lot of collector interest, it is most probably not worth what I have in it in terms of resale. but it looks pretty sharp and it rides very well.
The Merckx Professional I bought precisely because it was "all original" (save the saddle and tires). It even came with a replacement roll of factory bar tape and original Silica frame pump. This bike is in VERY presentable cosmetic shape, and perfect mechanical state. I could easily sell it tomorrow for well more than I paid for it, and eventually that is what will happen to it. Keeping in mind that vintage "all original" Merckx's DO garner collector interest, it'll be a cold day in hell before I change out anything on this bike without CAREFUL consideration. Besides, it's a pretty snappy ride as-is.
I think it is important to make a distinction between bikes that are in demand, and those that are collectible. For instance, I could sell any old crappy Peugeot around here for at least $100 as long as it is ridable. Run of the mill vintage Treks are in demand as well, and even mediocre examples sell well. But, they sell well because they have a reputation as good, solid, usable, well built bikes - not because they are rare or collectible.
As for the bikes I ride - well, I side with USAZorro. I equip them as I see fit, to suit my tastes and intended use. That is why that pretty vintage Palo Alto frame is going to get built up with modern Campy 10 speed. I'm gonna ride the ***** out of it, and I want to have fun doing it.