I use clip wherever players and vehicles aren't supposed to pass (such as treelines, border fencing, etc). Clip also comes in handy where you don't want players to get hung up on detail work or for players to poke through a thin wall.

Vehicle clip is good for keeping vehicles from getting hung up on odd terrain. For example, a low (waist height) stone wall where vehicles tend to get stuck while attempting to drive over them. I'd put a vehicle clip brush down the middle so infantry can cross, but vehicles have to drive around.