See, you are missing the point. Battle Regimen and buff enhance your spell damage one time only, and they have minimum 30 sec CD for buff and XX sec depends on how good your party is. Sure you can play with Banish/Scourge I at Ogre for 1500 dmg fully buffed BR or play with a deadly 2k dmg fully buffed BR Banish/Scourge II (on comparison, a normal non-buffed Banish/Scourge I is around 220). The 30% difference get much more noticeable with all the buff, much much more.