I agree with everything that has been written - practicing slowly is absolutely valid and important. Just, for curiosity, how much do you practice slow, and how much fast?
depends on who i practice with. if its with more advanced dojo mates, we go hard; i take the advice of my sempai...don't hold back if your partner can take the ukemi. if my partner is less experienced, then i won't 'go fast.'
if that hasn't answered your question, i think its because its hard for someone to say 'i practice 70% fast and 25% slow, and5% of the time, i'm sitting in seiza....'