Also, on the occasional listings of the 2005 COA 50c coin on its own can go for way higher than one still in the PNC - dunno why, and this happens quite often for other coins liberated from sets
I put it down to a quirk of human psychology.
We don't like to destroy things which we perceive should be precious, and an intact "set" is perceived as being more precious than one that is broken up. So we don't want to do it.
As type and date collectors, however, we want to keep the coins from the set with all our other coins, so that makes such coins still sitting in their big bulky sets wrappers less desirable to us - and, by the law of supply and demand, reduced demand means reduced value. Nick01 has expressed this sentiment in the OP...
Shame I can't put it in its slot because it's in a 'First Day of Issue' envelope.
Why "can't" you? Why "can't" I do the same thing? The only thing stopping us is our conscience, telling us not to destroy precious things. We know (or we should know) that such coins sold separately had to have been "liberated" from a set by a prior owner - probably by the same guy who's selling the coin to us - but as long as we didn't actually see the liberation taking place, it doesn't affect our conscience.
In effect, when we pay extra money for coins liberated from sets and PNCs, we are paying someone else to do our dirty work for us.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis