[sciurius]

I presume a main advantage would be smaller file sizes as compared to .PDFs?

Not specifically. While a .cho is on average 30 times smaller than a PDF size does not matter much these days. For example, the Google Play Services app is 222MB, equivalent to some 6000 song PDFs.

More important is that the song is transposable at view time.

And tunable. Each person of a band can have her own private settings that control the display (e.g., no chords for singers, big chords small lyrics for harmonists, a capo for guitarists) while still using the same song source.

But, while MSPro does a great job with ChordPro files, it is not yet supporting all the features of ChordPro. However I'm confident MSPro will catch up given time.