yes it is a lot easier to patch a game with no players on it. Teh game is "offline" only in the since that it is not taking active connection from payers. In any development enviroment there are dev servers and live servers. They need to just fire up teh dev servers to make th ecoding. once they feel comfortable then they can apply it on the live servers. With no active players they can patch on teh fly. So yes having the servers "down", no active players on it, makes it easier to patch.
I never questioned the fact that they use dev servers, that's a given.
With no active players they can patch on teh fly.
Where are you getting your information? Many types of server-side code you actually can patch on the fly(I'm a programmer and I have to do this crap almost every day) The changes within the server happen so quickly it doesn't even matter. Once a variable changes on the server, the client starts getting the new variable. Worst case scenario they release a patch requiring a server restart and you're down for ~10 min. Big deal. They could even release it during one of their scheduled maintenance downtimes and kill two birds with one stone.
But you're suggesting they leave it down and work on it till summer, then let everyone run wild on three months of untested code. I understand they test their code on dev servers but its never fully tested until thousands of players have proven its stability. From experience its best to release updates and adjustments gradually, that way if something goes wrong you only have a week's worth(or so) of code to sift through to pinpoint the problem, if you don't already know what it is. Basically my point is that its not a huge inconvenience having players on the server, if anything its a good thing(for business too). Pushing huge updates is kind of scary(at least for me); multiple errors compound themselves and even obfuscate other errors. It's hard to tell what's broken when you've got a ton of broken things happening at once. whatever. tl;dr