I use Microsoft Visio a lot, in conjunction with C2, for design and diagramming. I find using the Shift + Mouse Wheel combo very useful, especially when I'm flipping back and forth between the two. I find using the space-bar combo to not be as easy.
I don't see what's wrong with adding a bit more functionality to the editor. Shift + Mouse Wheel is used in a lot of other applications for horizontal scrolling, including most web browsers.