I suppose that depends a lot on the copter and what the owner wants to use it for. A very small machine, like a 250 racer, has a very limited amount of deck space so on a boom is possible but a very small cam on the frame is more likely. A racing quad would put a camera on a boom at pretty high risk.
For those that want to record video from an FPV cam (there are some very good FPV cameras available) the thought of seeing much of the aircraft might be quite bothersome. If using a larger MR, say 400mm and larger, putting the camera on a boom could be worthwhile as long as airframe vibrations did not get amplified by the boom and render any video difficult, if not impossible, to use.
For others the FPV camera is used in conjunction with a dedicated video camera. FPV is used for directional guidance and telemetry via an OSD while the dedicated video camera is used for still and video captures. Those that desire to produce a high quality visual product do not want to see any part of the airframe in the final product and go to great length and expense to assure the camera's view is only "seeing" things not associated with the copter.
If just flying around and having fun with an FPV camera there's no detriment to seeing some of the copter, like the prop tips, in the view. You're genrally more focused on what's further out front and leanr to ignore props and such in the screen.
