Yea im not sure why they added it to syphon strike tbh, to add more choice perhaps? i very rarely get the opportunity to DA my syphon strike at 70 because i have so much more to spend it on xD
personally i think that more choice would be better served in something like the form of a Dark Passenger that has a watered down Scourge DoT so that it would still be viable in single target aswell and also give more options in regards to how you spend your MP, while also adding a small layer of complexity by adding back in dot managaement to some extent.