Why do you need wheels to rest a stand-off on roof tiles?
IMO and IME a stand-off isnt just about making parts of a job more easily accessible it is there in addition to make it safer, wheels will not do that, they will make the whole structure you are standing on mobile! I wouldnt want that.
A stand-off without wheels then has more points of contact on the building.
Windowswashed what would happen to a ladder fitted with a stand-off with wheels resting on a window-cill was to slip an inch or two from the bottom?
I know from experience what would happen if a ladder fitted with a stand-off without wheels resting on a cill was to slip from the bottom. Id be interested to see the consequences with wheels!
A stand-off is a safety device, not just to help accessibility; unless its got wheels, then its a roof-ladder.