Personally, I tend to think that one needs to start out slow and develope a certain level of mastery of a technique before speeding up. This is for several reasons:
To avoid injuries.
To train the body in the finer points of the movements in the technique.
To see subtle mistakes that are being made and allow for more fine tuning.
Couldn't agree with you more. To add some I read in some books (The Dynamic Sphere I think) that training in high speed will make you easier to lose control over your partner. I don't suggest to train on slow speed, though, just train in reasonable speed, where your partner can react fast enough but still have time to think about the technique, later on after we have some experience we can do faster and faster.