The main reason to do that is licensing. You either need to get a redistribution license, or the client needs to work out their own licensing deal -- and either

The main reason to do that is licensing. You either need to get a redistribution license, or the client needs to work out their own licensing deal -- and either way, you don't want to be beholden when negotiation time comes. That also applies to MySQL, because it's GPL. That does not apply to Postgres, because you can just ship it with the app. I exaggerated a little, there certainly are times when you just don't care what database system you use and would rather have simple migrations at the expense of owning more code. But I think these reasons are weighted much too highly, particularly in the context of postgres.