You can often get away with some downtime, but that's not the same as not spending any engineering effort.

What kills you is when you take an hour scheduled downtime, communicate this out, get everyone on board... and then are down for a day. If you don't have a good, well-rehearsed, plan, you might be unexpectedly screwed even then. As a rule of thumb... something usually doesn't quite go how you expect it to!