Well, from a security stand-point, ActiveX controls a pretty well a no-no. It's fairly easy (and ... well, people do, some signed DRM ActiveX controls have) to write code that basically says "execute arbitrary code on the system" and other ActiveX controls can call that, if the code they are calling is signed. It was the major cause of a large lawsuit UK universities took out against Sony BMG actually.

If we're reviewing feature requests from the perspective of a different rendering engine similarly, ActiveX control use throws up more issues there-in.

I think the point I'm getting at here is: ActiveX controls are baaaaad, and you should find an alternative means of achieving what you want in the client-side, if possible.