That's Tumblr's equivalent of commenting. I find that it's much more minimal than normal comments, while still letting people respond to the author.
It's not as good, mind you, as showing no response to your work at all and letting it exist in and of itself, but people still use blogs as conversation, and I think reblogging's a much subtler approach to the problem.
The problem with this approach is that each person has their own picture. On a very popular page, this has the tendency to make the page very slow as it loads another 100+ items on the page yet adding nothing to your experience.
I'm here to read your content, not marvel at your e-dick-size.