As in the feed? Probably not.
I am not against them in principle. Thumbnails make consumption of some content easier. However, that's something that we have chosen to avoid as a design decision. We don't want to encourage content that can be breezed through.
Many places on the web do it very well, and I visit some of them. It's just not the experience we want to serve here.
I hope that's not a deal-breaker for you, but that's our game. It's not lost on us that we'd probably grow larger and more quickly if we did.