I'm a fan. Like the submarine analogy. Mattering what happens with physics I could imagine a situation where heading through a hostile area you would want to instead run silent and just let your ship continue to drift. In that case I'd say power down shouldn't take as long as powering up, i'd also say while running silent you wouldn't be detected as a ship. We have other stellar phenomena planned so it might be interesting if you appeared randomly as one of them mattering the size of the ship. Scanning we could go either way, say it detects objects in space, or detects heat/electrical signatures. I'd be inclined to go the latter.
I can say from some of the things planned for the future, there will definitely be times you'd prefer to use this feature.