I use Finale, so maybe the others are different, but in Finale you can't open an XML file, you must import it. Perhaps that is an option in Sibelius or Musecore?
I work regularly in both MIDI land (Sonar, Studio One) and Score land (Finale, trying Notion).
My workflow is dependent on the project, but falls into two broad categories:
1) Sometimes I start in Sonar, record audio, add MIDI, record MIDI, add audio, and while I certainly think in terms of conventional music scoring, I'm not working from standard notation. And sometimes, when I am finished, I realize that I need a conventional score. UGH! I find it easier to quantize and move things around in Sonar, so that's what I do - to a COPY of the project. Then I can export the XML, and import into Finale, where I spend (way too much) time touching it up. It works. I wish it worked better.
2) most of the time I start with pencil and paper - or Finale these days, it helps me think through the part writing. Once I've reached a point where I want to hear it I export the MIDI and import it into Sonar, where I assign tracks and finally get to hear my masterpiece. Needless to say, it tends to sound mechanical. If I am producing a practice tape I pretty much stop there. If I am trying to produce a recording I will print the score and play the parts in, and then I'll still spend too much time touching it up.
I'm afraid that's the deal, working in the two domains is not yet easy.
I am curious about the claims made by Presonus - so far I'm not getting it, but I think it is worth a little more effort, just in case they have really figured it out!