GL is also a 12-second buff and it does a lot more than Twin. If you use ToD/Frac on your rotation and there is a risk of losing Twin's buff before the next, you can use Twin in lieu of True. We should always prioritize our buffs--GL, DK, and Twin--over all else.

Again, I need to underscore that things are different in practice. Sometimes the right thing to do is use DK from Titan's front because Opo is about to wear off and he just finished his jump - the MNK calculus dictates it's better to have DK on even though you're dealing 100 potency instead of 150. There will be periods where you are not directly engaged with anything. While I'm willing to consider Zen-like execution of MNK skills down to the hundredth of a second, ARR content does not consist of fighting training dummies. My comparison of ID to strict MNK rotation was simple for the sake of illustrating how ID is not leaps and bounds ahead of the ordinary with no other factors considered. When you do introduce those other factors, ID becomes less and less attractive.

One thing I never considered is whether Twin/DK receive their own bonus on use or if it only applies to subsequent attacks. I will check it next time I log in if someone doesn't know already.