I'd say most clients use them most of the time.... but they're not compulsory.

They come in a wide range of styles but the oddest ones I've ever seen came with 2 Austrian clients early this year. They (the sticks, not the Austrians)looked so bloody akward it's not true. Setting them up (for me) was a bit like trying to do a rubic cube in a hurry, but these guys had them well sorted and they got the sticks up and the shot off, incredibly quickly.