I have the regular old Starbucks App on my phone.. seems pointless to have a secondary app (Passbook) that does what the primary app (Starbucks) does as well. I can upload more funds, pay for drinks, get free songs, etc all through my Starbucks app.. I dont see a need for another app to do the same thing.
Your observation is very valid. Furthermore, Passbook still need the Starbucks app installed in order to update its balance.