Many blogs display recent comments, either by the blog owner or other people. I've implemented this myself in the past, but now have reservations about doing so, for the following reasons.
Firstly, I'm not a legal expert but I should have thought that if you're going to display people's comments somewhere other than where they originally posted them, you should at least warn people that you may do so. A lot of blogs don't.
However, there is also a moral dimension: is it right to take someone's comment out of context, without at least some clarifying text? Perhaps most of the time this won't be an issue, but imagine this scenario:
Suppose I read on someone's blog that she wrote an article for a commercial magazine, for no pay in order to get her foot in the door, and has now been told that they are not interested in commissioning her for any paid work. However, because her article was pretty good they would always be interested in receiving more, just not in paying for them.
I might write a comment like, "Your first mistake was writing an article for free. You should always agree on the fee before putting pen to paper, as it were."
Taken out of context, that could be quite reputation-damaging. It suggests, for example, that I would only write an article if I am going to be paid money for it. Anyone reading the comment will not have the benefit of seeing the context in which it was made.
In this respect, automatically posting recent comments suffers from a similar consideration to posting Twitter conversations, ie they only make complete sense in context.
As for posting your own comments automatically, I don't see the point in that at all, unless it's to demonstrate to all and sundry how ubiquitous you and your wisdom are. But again, taken out of context, your own comments have little meaning in my opinion.
What I think would be quite handy would be an application that collates comments from all over the place on a particular blog post. I sometimes have few comments on the blog itself, but they appear elsewhere such as on Twitter of Facebook.
I think overall, my objection to automatic comment posting from an educational point of view is that it represents a poor use of ICT in education. To my mind, ICT should seek to solve a problem or answer a question, not be used just for its own sake. Perhaps if someone could explain the point of displaying comments somewhere other than where they were put in the first place I'd feel differently about it.