MSSQL is also available at any hosting environment I ever looked at. Although PostgreSQL might be available as well now everywhere, MSSQL was a no brainier for web at some point (unless you had extra money to invest in a database for your online app).

For most purposes all databases do fine. You really have to base the choice on how familiar you are with the product, and what other technologies work well with the product. For example, if you're a Microsoft shop, you'd probably go with Microsoft SQL.

This "good for X, it should be good for me" is something that bothers me, take php for example, if it is good enough for Facebook it should also be good enough for everybody, doesn't it?, sure, Facebook makes it work for them, but they spent a lot of resources to get there (they even made their own vm HipHop), that does not mean your startup has or want to spend that kind of effort and resources with it having now potentially better alternatives under your given use case?