As for myself, I don't collect covers. So if the cover
is in good condition, I will try to sell it. But if the
cover is in some way flawed or there are many on
ebay and
they aren't selling, why take the time and also pay to
list it.
So if it's common, not in great demand, you can use the
stamps but have no use for the cover, you aren't going
to try to sell it or trade it, then see it as a source for
what you need. What, are you going to hold onto a cover
you don't collect simply because it is a cover and buy the
stamps, instead?
I have soaked many FDC's, mostly foreign, since that is what
I collect, if it was dirty, wrinkled, creased, torn, or if
no one on
ebay was willing to pay 99 cents to have it. Stamps
from FDC's give you such nice used copies compared to the
normal run of the mail.