SE said they wouldn't add stances or spells that buff one aspect of magic and reduce physical abilities or vice versa. They also stated they wouldn't want to add abilities or spells that add mutually exclusive enhancements to either physical or magic. SCH is the class that chooses between white or black magic speciality. RDM, SE said doesn't choose between white and black magic or physical, RDM was supposed to be able to access all of it simultaneously albeit weaker. So SCH gets stronger WHM spells and BLM spells because they have almost no physical prowess. Since RDM has better physical capabilities, we have weakened spells or whatever. So to sum up, SE would never implement a stance where a RDM's abilities in one aspect get boosted at the cost of another(i.e like dark arts/light arts/hasso-type abilities where Phys+/Magic- or Magic+/Phys-)