But we, the consumers would lose. Without a healthy competition, there is no pressure to lower prices. And, there is no pressure to innovate on the existing iPad for Apple. So, yes, I would love to see many tablets - some with an Apple OS, some with Windows, and some with Android. What could be better than having the choice?
Microsoft produces really interesting stuff (such as Surface), and they've made good programs, but they seem to have quality only in small things. When they go to building large systems, layers of execs get involved and turn an interesting idea into a shoddy product.
If MS was writing iPad and AndroidTablet apps, items that were small enough to escape attention from all the Dilbert-like pointy-haired-bosses, and put together teams of 5 or 6 people to work on said projects, they'd probably make a lot of interesting things that worked and bring in some money.
But with Ballmer saying "Our new tablet is a huge deal and needs to be an iPad killer!", what that means is that every manager who wants to be an Important Person is going to stick his fingers in it somewhere, and they'll end up a confused mishmash.
Here's a real example of that happening: http://moishelettvin.blogspot.com/2006/11/windows-shutdown-crapfest.html
And here's a comic look at how things at MS can start great and end up with too many cooks spoiling the broth: http://www.youtube.com/watch?v=aeXAcwriid0
I guess what I'm getting at is that Microsoft is more likely to produce something worth using, and thus more likely to contribute to healthy competition, if small teams are building apps for other platforms. If they're trying to build a platform on their own, that'll be a Very Big Deal down at Corporate HW, and the first dozen revisions will have Clippy-like abominations in them, or be Vista-like bloated disasters, or just be Windows95,98,ME-like bugfests.