The problem with wheel-to-zoom is that it breaks the mac touchpad experience (and presumably other trackpads that support gestures). When you use two-finger scrolling, you move the page down until the mouse cursor happens to run over a map section, then the page stops scrolling and you start zooming. It's really annoying.

We explicitly turn off wheel-to-zoom. I suspect most sites do it for the same reason.

Most of our "real" users will probably pick a reasonable boundary around their home and leave it there for their lifetime, so disabling the wheel is probably the right decision for us. If our users did a lot of zoom navigation, we'd probably go the other way.