At OSDC Mary Gardiner gave a talk titled The Planet Feed Reader: Better Living Through Gravity. During the course of the presentation she expressed the opinion that short dialog based blog entries are a sign of a well running planet.
Certainly if blog posts respond to each other then there is a community interaction, and if that is what you desire from a planet then it can be considered a good thing. Mary seemed focussed on planets for internal use rather than for people outside the community which makes the interaction more important.
However I believe that planets are not a direct substitute for mailing lists. On a mailing list you can reply to a message agreeing with it and expect that the same people who saw the original message will see your reply. Blogs however are each syndicated separately so a blog post in response to someone else's blog should be readable on it's own. A one line post saying "John is right" provides little value to people who don't know who John is, especially if you don't provide a link to John's post that you agree with.
On Planet Debian there have been a few contentious issues discussed where multiple people posted one-line blog entries. I believe that the effective way to communicate their opinions would either be to write a short essay (maybe 2-3 paragraphs) explaining their opinion and the reasons for it, or if they have no new insight to contribute then they should summarise the discussion.
I believe that a planet such as Planet Debian or Planet Linux Australia should not only be a forum for people who are in the community but also an introduction to the community for people who are outside. AOL posts don't help in this regard.
One final thing to note is that blogs already do have a feature for allowing "me too" responses, it's the blog comment facility...
PS Above is a picture of day 59 of the beard, it was taken on the 5th of December (I've been a little slack with beard pictures).