Note that SingleThreadModel does not solve all thread safety issues. For example, session attributes and static variables can still be accessed by multiple requests on multiple threads at the same time, even when SingleThreadModel servlets are used. It is recommended that a developer take other means to resolve those issues instead of implementing this interface, such as avoiding the usage of an instance variable or synchronizing the block of the code accessing those resources. This interface is deprecated in Servlet API version 2.4.
Baically to be thread safe you need to consider all threading issues, and since the Single ThreadModel doesn't really provide anything except a brute force attempt at this, it creates the illusion of thread safety and needs to go.