They can technically be viewed as both in the Metroid series, seeing as an item like the Screw Attack is both required and extremely useful broken (in a good way).
I was referring to things like Seeker Missiles, Diffusion Missiles, and the
Prime series' Spider Ball, to name a few. Not one of those power ups is useful outside situations where the developers specifically intend them to be useful.
Or, to stay on the topic of Mario, take the Rock power up from
Galaxy 2. Not only can it not be found in areas other than where it's specifically required to pass, but you wouldn't want to use it in most areas anyway because its ability set is tailored specially for the aforementioned areas.
Developers seem to have to go out of their way in terms of design to make power ups useful. I much prefer them to be generally applicable.