Any reasons at all *not* to use MySQL in this scenario
Here are the cons that I see, this being a somewhat subjective question:

There are political reasons for and against this setup.

I don't know if I can be any more clear than that.

I would think this would be a huge benefit, so I would use this as my main selling point, because otherwise I don't see why not use the existing infrastructure to do what you need:

No need to migrate export data from MySQL to a new DB

Note that that's really going to turn into a con for you pretty quickly, cos everyone will say "just put a stub in that converts the MySQL requests into MSSQL" and that's going to be the negative all over again.