... and if you know how to cook, every mother would love to have you for a son-in-law.
What kind of knowledge are you talking about? How deep is it? Are you good in administering all of those databases? Do companies need an all-round expert, or do they mostly use one database? Large, but really large companies might use both Oracle and SQL Server, but - where's the point in paying lot of licencing money for different products?
I don't know, I'm an ignorant regarding all but Oracle (and I'm not particularly good at it either), so ... I'm probably not the right person to answer your question.
matman. It will always depend on what you are doing. Are you a general DB Consultant ? Some basic know-how for other DB products is ok but I recommend to fully stick on one product. From a market perspective Oracle is top and there are not many consultants on stage in these days.