I know in halo you only have two weapons, for most, a general weapon switch should suffice. After all, on controller you just press Y to switch. But on PC, I personally like to have specific inputs for my weapons. For example, I have my first weapon be scroll wheel UP and my second one be scroll wheel DOWN. But with the current way it is, if I scroll too fast or far, I end up rapidly swapping weapons in the heat of the moment, and often end up back on the weapon I wanted to switch off of.