The things that people will do to fill the spaces in their pre-printed albums. Browsing through this week's Kelleher auction, I saw multiple multiples broken down into single stamps. A used #78 that comes with a certificate from when the stamp was once a nice used pair with matching fancy cancels.
A hinged 73 single, with acceptable but not great centering, that comes with a certificate from a mint block. (Geez guys, the single stamp has a CV of $350 and the mint block is $2750 (687.50 per stamp). The single stamp(s) are now worth about half what they were as a block. Opening price for the single is $120 and not worth a huge amount more than that, IMO.
I found the block where this stamp came from on SAN, it sold at the Rumsey auction last April for $850. About right. PF graded the block a VF 80. One or two of those other singles must need to grade at least a 90 to recoup the cost of breaking up the block, but I don't see it.
The Kelleher sale also has a 163 (15c) mint single with certificate for a block, and a 166 (90c) mint single with certificate for the block it came from.
There is a beautiful 191 (90c) mint margin block that is fortunately still intact as a block. (Although when I scroll down the page further I see the certificate that it came from a mint block of 8!) Someone must be downsizing their collection.
Fortunately I didn't see any examples of single stamps that came with a certificate for the cover.
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I can't imagine a case where the stamp alone would be worth more than the stamp on cover.
I thought of another example.

Recently someone on this board had a big pile of first day covers and wanted to know what to do with them. They are bulky and not worth much to anyone. There was a suggestion to soak them off cover and end up with a large group of lightly cancelled stamps that would probably be easier to sell (and cheaper to ship) than the first day covers. Are the single stamps worth more than the covers? Possibly yes, in terms of cash flow.
But in general, I totally agree, it is better for the future generations of collectors to keep things as close to their original form as possible. If you only collect single stamps, then only buy single stamps. Don't buy covers or blocks and break them down into single stamps, leave them for the collectors who want the covers and blocks.
Even modern covers are going to be collectible someday (we hope there will be future generation(s) of collectors). The 66 cent rate was in use for one of the shortest durations in history. In 50 years it may be a challenge to find a properly franked non-Forever stamped piece of mail for each of the recent rate increases.