Jason : very interresting post !
I wouldn’t be able to discuss this on the technical "code" side of things.
This being said, I think the NewsListing snippet has a huge potential. I say this with a good knowledge of what WordPress, Textpattern, Nucleus or Expression Engine can do. While it sures lack some feature (but that will change), it’s incredibly powerful and gives you a lot of freedom when you want to customize your blog "the next step". I have no doubt a snippet will handle trackback, the UserComments snippet will get better, safer and richer feature wise. The user synchronisation with SMF is already a pretty nice feature demonstrating the real power of a framework such as MODx. The things you say with your WP experience certainly confirms this....
For this reason, I think MODx could become a hot tool for power bloggers such as John Hicks or the like

This being said, more casual blogger will definitely enjoy having WP, Nucleus or any blogging system integrated into MODx. The whole logic behind MODx makes this easier than it is with mere CMS, another example of the framework’s power. That’s why I also have no doubt this will happen sooner than later.
But personnally, I think in a few months, most will choose to go with a NewsListing/UserComments/Trackback combo rather than use an integration... flexibility is priceless