If you want the mouse wheel to reassign your left and right mouse click you can already do that. However, it's not bound by default because most people like to use middle-mouse button as a third spell and having that bound to the mouse wheel makes that incredibly fiddly.
Gonna have to poke your baloon here for a moment: "most people like to use middle-mouse button as a third spell". Really? Where did you get this info?
1. Watching hundreds of players at PAX East do that without our telling them to.
2. Getting dozens of bug reports from people accidentally moving the mouse wheel when we made the mouse wheel bound to the switching of the LMB binding during very late beta, with most people confused as to why this was happening given that they were using the middle mouse button to use some spell or other.
Am I usually just making wild baseless claims on what large numbers of people do or don't think?
I for one find the default scheme clunky and am very happy that there is now the other, genre standard method. When you find your game compared to Terraria and Minecraft (which, I will note, both use the exact same control scheme) you really need to consider the differences in controls.
Again, you're basing that on a population of one so far as I can tell. We have a lot more data on it. Many more people play WoW than Minecraft or Terraria, and so the more WoW-like controls seem to be a more natural fit for the default. This is just based on showing it off at Minecon and PAX East and also having six months of beta.
That doesn't mean the defaults will work for everyone -- clearly not. But that's the whole point of why we go out of our way to make our stuff configurable. Bluddy will come on here soon and tell you that nothing other than keyboard-directed aiming is a playable or fun way to play. There are others here for whom anything that doesn't involve a gamepad is just terrible. Etc.
I also can't speak much for Terraria, not having played it. But with Minecraft the whole thing with needing only one active item/weapon at a time is pretty logical. It's construction-oriented, and when it comes to combat there are like two options. That's not a slam -- I adore Minecraft. But its goals and AVWW's goals put them in pretty different genres, I think. Terraria is somewhere in the middle, but my read is that it trends more toward Minecraft than AVWW.