vehicle specific internet forums tend to bring together people who are having problems. This often makes it seem like the vast majority of people on the forum are having problems, because many people first find the forum looking for a solution to their problem. Many people would never look for this forum if they were not having problems. Plus, you wind up looking at a relatively small sample size.
So, IMHO, the percentage of any malfunction is higher on a forum than it would be in reality. For example, look at the MK3 forum problem archive, you will see a lot of threads about transmission and MFT problems, many of them from people looking for solutions. Many people believe that the percentage of MK3 focus' with these problems is higher on the forum than in reality.
I believe that is the case with the SPI motor and the dropped valve seat. (now let me say, I know I am in the minority with that opinion.) Add to that the fact that this is an otherwise very reliable motor, and this issue, which is the only common issue to our motors, seems like it effects every single SPI on the forum.
People with more mechanical knowledge than me and with first hand experience have said every SPI will eventually drop a valve seat. This may be true, it may not be. I do not know. All I know for sure is that mine has not.
If it really concerns you, you can replace the head with a modified one and prevent the failure, if you choose to. I have chosen not to on my car. People have told me I am sitting on a ticking time bomb, and that may be true.
I am gambling that my SPI will continue to run until I replace it (which for me will probably be next summer, when my wife's car is paid off). Basically, I looked at the cost of replacing the head, compared that to the cost of repairing a dropped valve seat, and decided that with the miles I drive, and how well I have taken care of my car, it was worth the risk to me.