As someone that uses a LOT of LH bass on my arranger, I find multi button pedals indispensable, but unless playing seated, volume or expression pedals are hard to work at the same time as sustain pedals!

There is an important consideration about expression pedals that few arrangers (if any) take into account... If you work the pedal for one registration, and then change to another registration, most arrangers' values 'jump' to the pedal's actual position, whether you want to or not. Some workstations (Kurzweil in particular) allow you to program an 'entry' value AND an 'exit' value that, when you change registrations, whatever the pedals control will be set to that value, regardless of where the pedal was physically left at from the old registrations.

It was a bit more to program, but it often saved you from awful jumps in volume or whatever else it controlled when you changed setups. I wish arrangers paid this much attention to practicalities...