It has been said numerous times;
The argument here isn't that you should own an album so that you can look at the cover. The cover art was one of many things mentioned in passing, and people kind of went off on a bizarre tangent, focusing WAY too much on that specific element.
The point was the sentimentality related to owning a specific copy of the album. A specific copy that holds value to the owner for emotional, non-tangible reasons.
For example, my copy of Octavarium is the first DT album I ever bought, and they're now my favorite band. For me, that copy holds significance because of that, among many other reasons. If I lost it, I could easily buy another copy, or just stream it online, but I would be a bit bummed out because that copy, partially because of the music and partially because of other elements, holds sentimental value for me.
My girlfriend bought me a Stevie Wonder album once. If she'd just recommended the songs on it, and I'd streamed them, it just would have been some good songs that I'd probably but not definitely have remembered, and possibly listened to later. Instead, it's an album with sentimental value.
If someone just flat out doesn't hold sentimental value in things, that's weird, but I can't argue that. That is what I was talking about though, not whether or not album covers are all art unto themselves.