The reblog function allows some sort of commentary but also puts a cost on the commenter. I can reblog somebody’s comment, but my response now goes on my tumblelog with all the other things I put there. If I have something valuable to add, I will add it. If all I have is something stupid (like the guy who left a comment on an article about egg salad wondering why he had wasted five minutes of his life reading about egg salad) I won’t bother leaving a comment. It is sort of like a comment—but I have to have it on my tumbelog for everybody to read. By increasing the cost of leaving a comment, reblogging eliminates comments left by people who know they are stupid.