Most times, when a company is doing server maintenance, it is to improve the stability of the servers for the tens of thousands of people that are on them daily. Software engineers are usually working every day to try to fix bugs. However, it's usually more complicated than most people think. Fixing one bug can lead to dozens of others being put into the game, which in turn produce more bugs when they're fixed, and so on. I know that a lot of other popular games face the same problem, but the limitations of the capabilities of software engineers are not endless.
I've definitely noticed that the servers have gotten more stable over the past months. When the game first came out, I was having trouble playing multiplayer all the time, but now, I haven't experienced a problem like that in over a month.
However, there are still definitely some bugs that need to be worked out in the game. I'm sure that with time they will be ironed out, but until then, all we can do is wait and hope that the people working on fixing them can do their jobs well!