Here are the scenarios I care about when it comes to blogware interop:

Background (these are scenarios, I'm not actually in this situation!):

I run a blog. It runs ok, the software misses some features, but I'll make do, because I have so much invested in it - X number of incoming permalinks as parts of conversations and trains of thought, plus hundreds of comments and trackbacks and categories and tags.

But then one day I want to move all my blog content, comments and trackbacks elsewhere, this could be for a number of reasons:

1. The blog is run by my employer. The content is mine (agreed with employer and employer is happy to allow exporting of content) but I'm moving job so want to my blog to move with me (I stress, this is a scenario, I'm not actually in this situation myself, yet ;-)

2. My current blogware provider has staled out on feature / service development. They are losing money and won't get acquired. The blog is about to die and disappear into the ether.

3. I want to provide alternative ways of storing the content for backup and offline / alternative distribution and / or apply new algorithms / operators to the content.

In the first two scenarios the idea is to move the whole shebang and keep it online so no content is lost, including comments, trackbacks and categories. The third scenario is based on the fact that it is my data and I should be able to do with it what I like as a matter of principle.