That might introduce excessively complexity to how it works. Having a server be there just for maint time would be a huge waste of resources as well for little benefit. Every MMO has maint and that is unlikely to change. Even the big name ones like WoW, FF14, which have dozens of servers to play on, bring them ALL down for their weekly maint.
It doesn't matter that patches come out less frequently then weekly for the CLIENT. We don't know how often they update the SERVER, which sometimes has not required a client update but they have mentioned it a few times. They could be installing server side updates every week during maint for all we know.
Also this week's maint is just under 3 hours, a massive improvement if they can keep to their estimates. NCSoft also doesn't have a tenth the employees Blizzard has and BNS is very much a 'winding down' project at this point. And it is more then just 'backups', there are a large number of tasks that they do during maint and backups are just one of them. Installing updates on their end is not like how we install things on our PCs, there are multiple different things running on the server at once that can and do need things done to ensure they work.