Riot Blade is only used for MP regen purpose when you need to Flash more. Using it as a dps source would simply drain you more of your TP quicker since you would be using 70>80 most of the time instead of 70>60x2. Because TP reg in battle is 60 every ~3 seconds, a simple way to say it is that you drain only 10 points every 3 hits with RoH, while Riot drains you 30 every 2 hits. You do the maths.