I think that if you want to understand why DRK changed in 4.x, you need to look at a single design decision. Specifically, you need to look at the removal of Darkside's MP drain.

In 3.x, there was a natural equilibrium between Darkside and Syphon Strike. As long as you were attacking something, your MP would slowly float upwards. If it got too high, either over time or because of BW, you could drive it back down into the middle with DASE.

The removal of the MP drain from DASE broke the equilibrium. Now, your tendency is to cap. DASE wasn't enough; so both Syphon and Souleater had to be capable of burning off a DA. Scourge was also a casualty of this; you can't reliably follow a rotation if you're forced to constantly make adjustments to your MP to avoid capping.

The resulting change also influenced our oGCD system. The number of oGCDs needed to be trimmed down to allow for increased DA usage. A lot of oGCD actions which previously depended on DA usage (like C+S) became forced double weaves, because you couldn't DA early in the cycle and pick up the oGCD after the next combo action.

Crater made an interesting suggestion earlier on with regards to MP bar size. Increasing total MP does influence burst, but it doesn't influence the rate at which you gain MP. What it does do, however, is give you more room to float your MP, without risk of hitting either extreme.

Either way, you now have to exert effort to stay midline, while previously in this happened naturally on its own.