Micro-blogs are blogging systems that collect your content (or content that interests you) into a simple readable blog style listing with no comments allowed. Normally these are used for content distribution on mobile devices. Can't a normal blog accomplish this as well and What other real usages can there be?

 Not to be bias I do have a microblog called the Socioblogger using the Tumblr system. I recently created it due to the fact my personal blog is down for maintenance while I upgrade the redesign.

simple content aggregation allows for this kind of "microblog" - point a WordPress blog at Google News/BlogSearch and viola, microblog. Pointing at this page from a mobile device gives you what I think you are talking about - for a primitive example please see http://facibus.com/onblogging/index.php/blogging-in-the-news/

What would you call Microblogging? Twitter or Tumblr? Or both?

I would consider both Twitter and systems such as Tumblr as microblogging. Microblogging is really more about distributing content versus creating communication. Although Twitter does not have Feed collection intregrated like Tumblr it is primarily used the same way in which to announce information where others can become "followers" or subscribe to RSS. Neither one giving the ability to response to the author or creator of such "microblog".

if that makes sense

Yep I guess that's a fair distinction... I guess the point is to distribute content like you say then. For me to bing content to the Attention of my friends without neccessarily getting in their face about it