I've always classed collectors as looneys but the mind boggles at that. I've still got all the tax discs for my first new car from 1985 till 2006 (when it was scrapped) - they're going to be worth more than I paid for the car at that rate.
When I sold my previous E36, It was sold with every recorded service / Recall / Tax Disk / the full cars details from the German Makers, the buyer was very well pleased that all the History was with the car, Each to their own I presume.
I can (sort of) understand people holding on to their own tax discs out of sentiment, but why would you want someone else's? For cars you have never owned?