I agree with you; Tumblr seems to fill a niche somewhere between a public del.icio.us feed and a blog. It lets you show people things in a genuinely unified manner (unlike del.icio.us, which is links, only links, nothing but links), without obligating you to go into commentary. It's kind of like a multicast, pull version of emailing something nifty to a friend. I think there's definitely a market for that.
In a world that is feeling the weight of information overload, I think it's great that someone is allowing people to just get to the point and "cut the crap" so to speak.
I'm not saying that blogs like TechCrunch aren't worth reading. They are. But I bet Michael Arrington has a lot more to say every day than he can write full blog posts about.