As a ‘footnote’, The MIDI-Compatible Part of MusicXML | MusicXML 4.0 makes for interesting reading, at first glance.
(1) The primary goal of MusicXML is clearly stated as being about sheet music and the interchange thereof. . But sheet music and notation is about documentation of some musical work, not the production of same. The key question in this thread is whether presence or absence of notation facilities in some music application is the key determinant of whether that application is 'professional or a “toy”.
(2) For the actual rendering of some documented music into something which can actually be heard, the specification is also clear " The only elements that are required, though, are the sounding elements that relate directly to creating a MIDI file from MusicXML." Read that again; as far as the the generation of the end product, MusicXML appears to add nothing standards related than the generation of a MIDI file - it’s about MIDI import, which Scaler does very well.
(3) If music creators wish to exchange the actual realisation of a composition, they can only do so by means of some binary encapsulation of audio or - by MIDI. Notation exchange alone does not help one jot.
(4) It was interesting to note that a Google search for “MusicXML and mpe” produced 1 hit; and it’s very hard to only get one hit from Google. So on the most important MIDI developments in 37 years - MIDI 2.0 and MPE - it is silent, in the public domain at least. Hard to tag that as being a standard which should be mandatory for a “real” product.
(5) So speaking as an amateur dubstep headbanger (says he, bending over to snort a line), and a ‘non-reader’, stave wise, to boot, notation doesn’t figure in my product selection criteria. Fortunately, music application vendors in the mass market (I’m not including AVID here) need us bedroom techno-grunge-EDM types to have an economic framework to survive.