This might be one of those cases where motorcycle techniques don't carry over very well. I've never ridden a motorcycle though, so I could be wrong.
I don't know anything about dirt bikes, but teaching one finger braking is much the standard with any professional mountain coaching/training in the US and Canada. (that I have been exposed to) What I was told is that with MTB disc brakes you don't need more than one finger for control. My suspicion is that is has to do with not overpowering your discs. You are much less likely to endo braking with one finger than with a handful.