The Express version is designed by Microsoft to fill this hole, where a solid RDBMS engine is required but not all the extra features. I guess encouraging the use of Express also provides a certain amount of vendor-lock-in too.
The problem is that Express has a limit on the size of the database. Occasionally you have a need for a decent data store without all the overhead and costs of an enterprise solution. That's where OSS RDBMS like Postgres and its ilk find their niche.