I'm with Hanna on this. Yes, one should work at a slow speed. Yes, one should work at full speed. Yes, the energy is a bit different, often quite different.
Why does there have to be a dichotomy? Or, why does this have to be so complicated?
I can't imagine anyone always training full speed. At some point, we all have to slow things down and really try to figure out what we're doing, right? At the same time, if we don't attack/get attacked at a fast speed, we never get the chance to test our understanding of the principles and techniques we practiced, right?