Honestly? No. Turning off Darkside cuts off a ton of your abilities, and the MP regen during combat doesn't outweigh the cost of re-entering Darkside. MP still regenerates outside of combat, regardless of whether or not Darkside is on. The only situation where it might be worth it is during very long phase transitions in later raids, where the MP regen might outweigh the cost of Darkside. And, of course, it's also a major DPS loss. Otherwise, it really should be up at all times.