My favorite factoid is that Windows desktop Outlook uses the MS Word engine to render emails. If regular emails are caveman html (which they are), then I don't know what that makes Outlook html.
You can copy+paste anything from any Office product and it will render in your email. It also makes doing mail merges much simpler.
So as a corporate email tool it's a smart idea. Making it harder for advertisers to design fancy emails is just an added perk.