I don't think there is really ever a time for slow reps. Squeezing and holding the contraction, sure. Isometrics, sure. Even slow burnout negatives, sure maybe occasionally/sometimes (even though I am not a fan of them at all). But slow rep speed--meaning lifting speed, no. I am firmly an advocate of thib's training philosophy in this subject. Only exception I can think of is brachialis vs. biceps braachi recruitment.
I really do think there is a lot of value in understanding slow vs. fast twitch composition, but I would think in terms of rep ranges rather than rep speed. Also there is a middle ground--like kakno mentioned there is a large chunk of the time where you really just want to train the muscles to perform your goal, even if that goes against "fiber makeup" because the fibers can take on behavioral qualities depending on your training style. I think the areas where slow/fast twitch changes are most important are pure hypertrophy and postural (think of endurance of stabilizer groups, or postural muscles to fix bad alignments).
If your goal is pure strength, or power, then it makes little sense to focus on higher rep ranges (meaning above 12) except just as a way to get volume in.