Most MMOs have hundreds if not thousands of quests, textures, user-interface elements, statistics, NPCs, zones, instances, skills, and animations. On top of that they try to provide an environment where thousands of players can interact with each other without causing noticeable client or server performance issues. Obviously, there are many complex pieces to an MMO and messing up something small can have huge unforeseen consequences.
Ironically, patches are intended to fix game issues but for some reason they often break something else in the process. No developer is immune to this phenomenon. We here at Massively play a wide array of different MMOs and we can all share tales of patches gone horribly wrong. We also know that even though most patches are player-tested before going live, it makes little difference.
So, do you play on patch day, even with the knowledge that you're likely to encounter issues like bugs, crashes, exploits, and possible rollbacks? It's a bit of a paradox really. If people avoided playing on patch day, it would simply delay the inevitable problems from turning up. Patch day would become patch week, month, or year. Patch days need mass amounts of players to uncover the issues. Unfortunately, it can be an extremely frustrating experience if the player doesn't have the tolerance for it.