It will teach them on the real-life business world of constant communications even if you're away from the desk. You need to be receptive of that these days now in order to do a good job and a Blackberry is a good tool for that. So it is not stupid. It may be a toy in other respects, but if the school actively uses email and Internet to all the students for distribution of things like homework etc, the Blackberry can easily be adapted into the existing infrastructure easily.
Also -- new additions to the class could be made to enhance the training of business-handling due to the provision of a business communication device. A business emergency simulation drill would be a good fun addition too, like a pretend CFO sending a simulated "unexpected urgent message" that an MBA must be able to behave in a prescribed manner to help administrate the emergency, for bonus points.
Therefore, this is not Blackberry training.
This is constant-communications training, with Blackberry as the tool.
It's a valid part of the modern MBA life.