The number notation looks good and could be handy, but I personally think implementing melody writing is not good for the app.
A gigging musician should know the tunes, period.
If she doesn't, she'll have better options with a small pentagrammed notebook than with a cramped screen with chords, lyrics and melody (even with ipad)...
Keep it simple.
My two cents from an enthusiastic fan of iRb and Massimo and staff's work.