One of the reasons why a manufacturer may choose not to use the 30 second ratings is that the transients allowed for this rating may not be as large as those allowed if you use the 2.5 minute ratings. (Go figure...)
For certification purposes, transients are not allowed to be used to demonstrate performance, and even for category A machines, you cannot use the 30 second rating for the takeoff engine failure portion (strange as this may seem - read the Part 29 sections if you don't believe me).
While this is a strange way to do things, it means that in a real engine failure you will have better performance than in the flight manual, which is a comforting thought.