People keep saying this but I fail to understand how. You're taking 140 potency of MP and turning it into 380 potency, the MP cost of which is paid for by the time you can even use TBN again. You're only delaying 1 SS/SE combo per minute, or half a dark arts and 10 blood, so you're not delaying your mp regen by any significant amount.
Provided you always have CnS paid for, I cannot understand how turning 140 potency into 380 as often as possible is a DPS loss.