Interesting question Joy
My guess is that the only people that might think about doing this would be the Oracle Database customers - but at that time, they have already made significant investment, which might make a switch not worth the effort. SQL Server users are unlikely to switch, unless they're starting to see far more users than they originally expected (over 400 or 500).
I know that some people have certainly made the switch - the question for them is "was it worth it" ?