But how would you drag it? Left/right for zoom in/out, or up/down? Or click/drag Select to zoom in, Adjust to zoom out? And when you start adding extra features, such as a click to open a zoom dialogue box, or to initiate a zoom dragbox, you're looking at something that should be on a button bar, not part of the window furniture.
I've seen applications that let you see the mouse wheel for zooming in and out, centering on the mouse pointer. Quite nifty and accurate for zooming in on a particular detail.