For many cases, portability is a complete non-issue. For example, at Pathwright we chose Postgres for a bunch of reasons. We don't care about MySQL or Oracle or SQL Server. If there ever came a point where we had to make a switch for some reason, it wouldn't be horrible to make the necessary changes to the smaller number of places that we use SQL.

Of course, portability could be more of a concern if you are writing something that you are going to let customers run on their own hardware/environment. Though even then, I'm not sure I'd want to have to deal with supporting customers using every DB server that Django supports...