I was playing DMC4SE on LDK mode and I think I understand why they decided to assign the launch and helm breaker to one button. If you play like me you tend to let go of the lock on sometimes to move around faster or when you got your target and are just messing around but with the huge number of enemies on screen the lock on is rarely going to pick the enemy you are trying to target and with the crowded situation back might not be where you think it is, specially in mid air. Finding the target you're locked on to and trying to figure out which direction is front and which is back takes up precious seconds and by the time you have the landscape's already change so no point in trying to look for it now that that's all different. It's frustrating and even inefficient.
I don't think I like the idea of a dedicated launcher at the sacrifice of a secondary attack function and I've always thought that DmC didn't need it, it doesn't have an overabundance of enemies, but maybe it's actually not as bad an idea as I thought before.