Why on Earth would I swap my RDBMS? Well, because IBM or MS or whomever offered a much better deal. I've heard that these companies (including Oracle, of course) compete quite a bit ;-)
What you do now can seriously impact how easy it is to switch, should your non-technical management decree it. If you take advantage of Oracle-specific functions, you can drastically speed up your queries, speed up your development, and hurt your ability to switch should the need arise. Kind of a trade-off.
Personally, though, while I understand the concept and need for the ability to switch, I'd still go with whatever gave me the maximum benefit under the current infrastructure, without impacting maintainability in a negative way, and then tell management what the costs were to switch our application for when other sales guys approach them to get us to switch. It still may come out as a benefit to switch because the switch itself is a one-time charge, while the licensing and support is an ongoing charge.