(First post, treat me gently...)
I'm in a similar position. I've been running my weblog on MT and I'm now evaluating WP. So far, I like what I see. But the question about exporting posts in a readily usable format seems to me to be a perfectly valid one. This is something that MT gets right - it's very simple to export all posts and comments into a text file that other systems (such as WP) can be made to import very easily. It's also useful when testing new versions of MT to be able to quickly import posts from a live weblog into a test one - which a database backup is not altogether useful for, given schema differences in newer versions.
This isn't (to me) about specifically exporting in a format friendly to MT, or any particular package, but about having a plain text backup of one's work. Someone who knows what they're doing with PHP and MySQL ought to be able to code this - perhaps it's a task for a plugin writer rather than the core developers, who as you say, need to concentrate on the core of WP.
I've seen the question asked a few times (and I've been looking at WP for less than a week....), so it probably is something which a significant number of users would benefit from.
Sadly, my cluefulness in the PHP/MySQL area is not likely to rise high enough for me to write anything like this in the forseeable future. Anyone else care to give it a shot?