You need BIS for all BB related services, such as Internet, email, BBM, App World, etc.. What do I need a Data Plan for? - BlackBerryFAQ
Many carriers also require you to have BIS before allowing WiFi access since you can't get WiFi without the proper service books.
In most cases, data will go via WiFI when connected rather than via the carrier's data network. Some apps may require a cell data connection so I cannot say that all data will go via WiFi when connected.