I have to say "it depends" on what the app is supposed to do (including "scale") and how specialized it needs to be. Having written a small and specialized "data engine", including my own sort routines, clustered and nonclustered indexes (non B-Tree), date/time functions, etc, etc, I might not even use a database engine. Of course, I'd also have to weigh out whether doing such a thing were cheaper than buying SQL Server and what the hidden cost of people hating proprietary databases might be. They might not buy the product unless it uses a recognized/established dtabase engine.