In my state, shoulders are to be used by drivers to let faster moving traffic pass when there is not an available passing lane. I use them as such and use bike lanes the same way. I consider shoulders more vehicular as shoulders are not intended to be a travel lane and thus do not direct cyclists to travel straight from a lane to the right of another lane where right turns are allowed. Shoulders can also be used by any vehicle that is going slower than the speed of traffic to allow traffic to pass. Additionally, shoulders can be used when a vehicle has mechanical trouble, for parking, by pedestrians, rollerbladers, and skateboarders, or for a bus stop (to name a few of the uses I often see).