The reason that Apple does not allow other engines in iOS is simple: it keeps all performance and security issues restricted to a single code base. That's good for users too. Considering that browsers implement the gross of their functionality (rendering + javascript) according to standards, there is no reason to have two different platforms that do exactly the same thing. As long as the standard is not owned by any private individuals, this is beneficial, and the same would apply to Windows tablets.
That said, leaving everything on the hands of Internet Explorer scares me, so maybe this fight should be supported :)