Good point about collectors; some don't ever play the records. Years ago I bought a collectible Mercyful Fate vinyl from someone and he was flabbergasted when I later told him that I had played it!
You're also right that the cover is an important part of an album, but is it really a $400 part of some singles?
Especially when, in some cases, at least half the copies were issued without a picture sleeve in the first place? In those instances, unsleeved copies are not
incomplete, they just don't have the 'extra' that the sleeved copies do. Nevertheless, most people will pay a lot less/are not interested in the unsleeved copy... they hold out to try and get a sleeved copy, even if it costs them hundreds of dollars more.
I understand going bonkers over the sleeve in some cases (if I ever buy the Marz 7" it WILL come with the awesome guitar-playing-alien picture sleeve!). Speaking as a small-time collector on a small-time budget, I can either 'settle' for an unsleeved copy OR simply forget about ever owning the record since the sleeved copies are usually out of my price range.
"I'm sorry Sam, we had real chemistry. But like a monkey on the sun, our love was too hot to live"