To be fair, many of the posts without thumbnails are low-quality spam.
The problem is really that there are many posts whose main focus is the text (such as my own) which aren't too low-quality (Not mine, then), and people who avoid posts without thumbnails miss them.
I mean really, would you avoid people just because they happened to lack thumbnails? I probably would, they'd look fairly horrifying, but with your whole evil clown fetish I can never know for sure.
I give scant regard for thumbnails!