I'm willing to bet that if I grab the current Mess code, and unzip it over my current MAME tree / any current SVN build, then all hell will break loose.
This means that unless I create a specific tree for working with Mess, which I don't sync to the MAME SVN, then I can't work on MESS. That means having to manually merge back in changes, and not being able to test the MAME/MESS side of things together in the same tree. It's also extra effort, and generally annoying.
The projects should co-exist in the same tree *somehow*, and should always compile, major changes should be made to both projects at the same time. Developing things which share code is more difficult than it needs to be otherwise.
For systems where it's appropriate MESS should have set lists like MAME in addition to the current open ended approach (which is ueful for homebrew & systems where the media isn't really 'fixed')
I don't dislike MESS (some of the recent changes have been very impressive thanks to Judge etc.) but it simply doesn't fit in my development methodology.
Sorry if others disagree, but that's just how I see things.