Only Oracle and Microsoft's SQLServer are closed source, and when something goes wrong and you have a problem the answer is just a phone call away (and cash if course). Anyways MySQL and PostGre have several enterprise consulting services but in the end these consultants aren't really resposible for the product, because the product belongs to everyone. Which is great because you can go in and fix the code if you are good with C and relatively lowlevel programming, but if you aren't finding the solution might become a wild goose chase.
Now since not everyone is skilled enough, and those enterprises with money prefer the security (in the business sense) of the closed source databases, is the reason why these solutions haven't gone out of business, besides the fact that their implementations are solid and worth the money if you have it.
Ok now finally the most important difference is between SQLServer and Oracle and that difference is the OS, most people using Windows will stick with, you guessed it, SQLServer, but if you run on flavors of Unix Oracle is your closed source solution. Anyways I use Oracle on Solaris, but if our target were Windows I would probably use SQLServer because both products are rock solid, but I trust Microsoft has some special tricks under the hood to get the best performance on windows.