Stupid Notch, his fascinations in silly things and downright messed-up ideas. If those enderpeople end up messing up landscapes and my creations by moving the blocks around and never putting them back on their place, I am going to be pissed. And either I will promptly end them by modding them out of my game, bringing their fiddling to an untimely end, or they will end my interest in Minecraft for good. (Hence their name: endermen, people who end things. No, seriously, what does "ender" even mean? I am obsessed with this word.)
Man, I liked this role-playing game, Gothic, with its clearly hostile world, but in Minecraft, everything, hostility is taken to eleven as almost everything is trying to kill you and screw you over in the most malicious way possible. It's getting tiresome. I probably won't be interested in befriending the native tribes of the pigmen, either. I just want to seek out Herobrine, the ruler of Minecraftia, slay him and enslave his minions.
But seriously, what the f**k. Those (sl)endermen sound like they are able to mess up compact mob grinders. Though I don't think they can spawn in 2 blocks tall areas, only 3 blocks tall ones. But still, since they can blink, they could accidentally end up inside the spawning chamber, where they could pick up some vital blocks, misplace them and damage the death factory.
Tl;dr: I am not going to like it if they actually start to move blocks.