For us, it does matter. We have conferences running all over the world which use us for their schedule - for them, two hours of downtime in the middle of their event is a big problem. Since we're a global site we have to think about events across every timezone so there isn't really an obvious time to do this kind of thing.

We might be able to schedule downtime for a period when no active events are going on (tricky considering the number of conferences happening around the world) but it's much easier for us to be able to make these kinds of changes without worrying about events that are using us to serve critical information. As it is, we still make sure to communicate planned read-only mode periods in advance so conference organisers have a chance to plan around them.