It's not just about fluff, it's about cohesiveness. My Blood Angels devastators can rage out. Does it add an on table benefit? Rarely. Does it make them overpowered? No. Does it make them feel like they are part of the same Chapter/Legion as the rest of the guys on the table? Absolutely.
If it can work with Blood Angels and Space Wolves, it can work with the cult Legions just as well.