There are, believe it or not, things that IE does better than WebKit. For instance, IE uses Direct2D (essentially the state of the art for vector graphics) for rendering.
They can rewrite the renderer in D2D.
Yet it doesn't use WebGL (essentially the state of the art for hardware-accelerated cross-platform three dimensional rendering in the browser).
Sure, there are lots and lots of things that WebKit does better than IE. I'm just pointing out that it's a tradeoff; it's not cut and dry in favor of WebKit.
Though I somewhat agree, having moz and ms using separate rendering engines does provide some opportunity for innovation.