How many web sites are going to require Windows Internet Explorer 8 the month it comes out?
None, because Microsoft is implementing an open standard, rather than someone implementing a Microsoft technology.
To "stick to" something implies that something has an opposite from which one should abstain. As I understood it, the opposite of "free APIs" is "non-free APIs".
Sorry, I thought I spelt it out enough the second time around. Lets try again: They should stick to providing access to APIs for Free software, where the important point is the "access to Free software" part, not the API.
GNU got started by implementing AT&T's UNIX APIs. Inventing new APIs to be deliberately incompatible with non-free software smacks of NIHism.
Dude, you need to check your reading comprehension skills. I said "access to Free software", I didn't say "reinvent crazy new shit to replace stuff that already exists". I even gave the explicit example of GTK+ and Qt. How is this NIH? Hint: it's not.
So, yes, Mono's VM is great, C# is a great language, Mono's implementation of the ECMA standards is great (assuming deployers don't get sued for any submarine patent encumberance, like those GPS guys (Garmin?) recently did) and so is the fact you can write great GTK+ and Qt apps using it.
However whenever they implement something to make it easier for people to justify the use proprietary software elsewhere, for example, Moonlight, then they are harming the Free software community in the long run.