His logic was that:
If a company is already paying for a commercial database, they would have no problem paying some more for someone to maintain it or develop on it.
I started thinking about what he said to see if its true. Not so long ago, I worked on a project for big car manufacturer in China and they used Oracle databases. They had the resources to hire quite a few PL/SQL developers for something relatively simple (but I am pretty sure those PL/SQL developers are still working on it as they get paid by the hour).
On the other hand, I used to work for a relatively small company that had a profitable website and they used MySQL. During my time there, I put a lot of work into optimizing the MySQL database, although it wasn't my specific job to only work on that.
Would a small company afford to hire a MySQL expert? They would probably choose someone who can do some MySQL and PHP as well.
To strengthen the point, a while ago when I was looking at jobs listings in the UK, the vast majority was for MS SQL positions then Oracle and maybe one or two for MySQL.
My own conclusions are that it depends on which environment you would like to work in. I noticed that medium/enterprise businesses tend to work with commercial databases and web companies use MySQL. So sticking to expertise in certain databases will force you into certain companies.
Thats not to say, however, that if you only know MS SQL, you will never work with a company that has a website. Quite a few websites use MS SQL (no where near MySQL though) and companies that use Oracle for their offices might choose to use Oracle for their website too.
Personally, I really liked working for my old web company. It was exciting, interesting and allowed me to gain experiences in many new fields (and it was also where I started liking MySQL).
Where as the enterprise company I worked for was slow, dull, very restrictive and very frustrating.
Can I make a connection between companies and which database they use to whether you will enjoy working there? Absolutely not.
For now, I will stick to be a part of the MySQL community, simply because I enjoy it. I will look for jobs that hopefully I will enjoy working there. It also wouldn't hurt to learn about all the other databases, if nothing else for my own knowledge.