I haven't really paid attention recently, but at a conference several years ago a former Microsoft sales rep said that nearly every "enterprise" product they sold required accompanying SQL Server licenses.
Most "enterprise" products are complex enough to need some complex backing store. A SQL server database is a natural choice for this in the MS world. Even TFS (source control and associated work item tracking, kitchen sink, etc.) is backed by one. This is not a bad thing, (e.g. you get atomic transactions from the DB), but it's interesting that most other source control systems are not backed by any relational database.
