A while ago, I got to witness a debate on HDs whether it was better to keep them up and spinning or was it better to turn them off when not needed.

For keeping them up:
No torque stress in the spin up or slow down. This is supposedly better for the mechanical parts. Also if you keep them running the heat/cooling will stabilize and this will stop unequal expansion/contraction of dissimilar materials the drive is built of.

For shutting them down:
Saves energy. Mechanical parts do wear out eventually from use.