A 7" exists because it does. It is as worthwhile and worthless as any other format. As a listener, the only thing worthy of consideration is whether the shit is worth listening to. If one's basis for that is the damn format, what can I say, people do things for inscrutable reasons... but so do chimps.
I respect and admire the reasons an artist or label may offer for whatever they deem fit, but when it comes down to it, I really couldn't care less.
From the label viewpoint, it seems apparent that market-based arguments tend to fall in the minority, and listener preferences, though good and well, are not all that influential. A 7" exists because it does.