[QUOTE]Originally posted by philipsmith:
Problem with them is that they need to be spontaneous BUT with perfect timing to be fully effective. That's why they're not taught as aprt of the syllabus. [/QUOTE]

Probably the bigger reason is that the ukemi has to be spontaneous, yet perfectly timed and executed to avoid piledriving your head into the mat. It's not something you can do without excellent ukemi.