I've heard the use of serial interfaces of router is not so common in the current network environment. But in some exams like CCNA, you can still see questions including the knowledge of serial interface. So, I don't know if it is true or not that using serial interfaces is not common.
As Ricky Beam has already written, even though Serial connections might not be common where you live, that doesn't mean they don't exist.
Over 90% of the connections we have in Europe use either Ethernet or xDSL. But in countries like Philippines, Nigeria and Cameroon, we have a lot of Serial connections.
In the end i guess it depends a lot upon where you are and what is available, that is also why Cisco have not ruled this type of connection out of their certifications. I do not know of any database specifying the most used connections based on which country you are located.