It doesn't have much to do with free vs. non-free. Just because
MySQL is free doesn't mean it's a great product. I do agree
with MS, MySQL isn't. It just lacks too much from what you
should expect from a relational database. While I don't touch
MS products with a 10 foot pole, I wouldn't trust any valuable
data to MySQL either. See also http://openacs.org/philosophy/why-not-mysql.html.