I guess they are expecting Timers to be available eventually in stateful session beans.
page 510: This functionality may be added in a future release of this specification.
Apart from that a Stateful Session Bean can hold a reference to a Timer (one that was created by a another Bean).
4.2.1 Instance Passivation and Conversational State - page 72
The container must be able to properly save and restore references to timers stored in the instance’s state even if the classes that implement the timers are not serializable.
I guess the container does not serialize the timer using Java's serialization mechanism during passivation lifecycle.
The container instantiates a new timer object, initializes the values of the timer when the timer is activated?