I am like you, not knowing where you'll be from day to day, and yes it does create a bit of a problem when booking hostels..but know this, that you can book hostels like1 or 2 days before you get somewhere, use the TravelPunk Hostel Booking ( Shamless Plug)
and check for availablility when you do know where your going.
As for the bus pass...is it alot of money??? Does it gurantee you a seat??? Are there any time or date restrictions??? These are all the things you gotta look for, I have heard that NZ is a great place to meet trav'las whop have rented or bought cars
( I have no firm prove of this) so you may hook up with people like that for a couple of days here or there, which would make you not need the bus pass. I guess if its not alot of $$$ then go for it, for secutiry sake, and if they have night buses then you might be able to crash on a few at night instead of paying for a hostel. There are ups and downs for sure...but go with your gut.