It's a good question, and of course all coins should be enjoyed (dare I say, even Churchill crowns, '67 pennies and even 1971 1p's eventually!). Some collectors actually prefer EF coins because they have seem limited circulation, almost like they have more of a story to tell. But with UNC coins, each one (especially the older ones), is in way, a small miracle. For throughout it's whole life people have been able to resist the huge temptation of spending it. But the reason UNC coins are always worth more is simply because they are rarer than EF coins, and there is therfore less supply to meet the demand. Perhaps with stamps, the UNC ones are more common? That would be the reason they are cheaper than used ones. I know nothing about stamps, and they do not appeal to me.