I'll chime in and say that Adobe has a lot more then just Flash. Let's not forget the entire Creative Suite, which is actually a very successful and well respected software system.
Apple has this reputation as being a good choice for designer types, now imagine if Adobe^H^H^H^H^H Microsoft Creative Suite suddenly only supported Windows.
So obviously there is some serious competitive value to owning Adobe. Now the question is how much is Adobe actually worth to Microsoft.