Not only will long stuff blunt your speed, but doing your sprint training too deep into exhaustion will also blunt it. I see people doing "sprint workout" where they're bogging down, their power is dropping off, and they're grinding out low-rpm work. That doesn't help your muscles fire harder faster.
When you're doing sprint work you should stop when you bog down. You should focus on the period when you are applying maximum power. You should try to extend it, but you shouldn't dig yourself into a pit teaching your muscles the wrong lessons.