The papal apartments "are already there and ready". In fact they existed before the Domus Sanctae Marthae.
I fail to see how this in any way saves money, or promotes the idea of “humility.” As you pointed out, ELS, the papal apartments have to be kept up whether he is living in them or not. And now the rooms he will be living in have to be kept up, as well. It will also be a huge headache for his staff, as anyone who has ever worked as a secretary or executive assistant well knows. Having the office and staff rooms near the Pope’s bedroom in the Vatican were done on purpose, so that his staff would have easy access to both him and the office (copiers, computers, phones, fax, files and filing cabinets, calendars, stationery, etc., etc.). Now the logistics will be a nightmare. Not to mention security. It will also be a huge imposition on the people who live permanently in the guest house. They will be subjected to security and all the problems that will go with the head of state living in their apartment building. On the contrary, I see this as causing a huge amount of discomfort for a very large number of people, and in that regard, it is displaying anything but humility. It seems quite selfish, to me.