Hello, the reason is quite simple in fact.
I am VERY happy with everything you guys have done, but I really think it is easier to make changes to a song on my Mac than on my iPhone.
So consider the following real situation :
I have a group of 4 people, guitar, bass, drum, singer, sax
Only one of us (me

) has an iphone.
I need to print the song for each member. Sax will have a different key.
Then as we practice the singer asks for a different key, then I need to reprint everything after making the changes on the Mac.
So basically it is for printing reasons.
Today I do this with Band in a Box, but I'd rather leave BiB and just work with iReakbook, which by the way has for me a better look for jazzers.
Of course if there was a way to send a song from iPhone to Mac then that would do it. Also a bridge between BiB and iRealbook would also be a solution.
I hope this is clear and answers your question,