Think of Mono as following the same process: we are bringing the best technology out there to our beloved free software platform. And at the same time it serves to be a magnificent upgrade on the development platform."
In general, we should seek to provide a free implementations of any facility that users want. I think we can presume that many users will want to run C# programs, so a free implementation of C# is a desirable thing.
That should be the basis for considering Mono or any other C# implementation. As long as it is free, it is more or less a good thing.