Why is mp recovery useless?

You mean you can hold the drain button and the MPR button down and just recover MP every couple seconds?

I still prefer oak barrel. Both for lulz and for certain situations.

Well I don't think you can use it as the same time as another skill, but you can use it while you're moving and it won't make you stop moving. The animation for it is also very fast and once you pres the button you can already start your next attack.

You can get oak barrel too. Max out oak barrel in 2nd job, then get mp recovery with your wasted shockwave points in 3rd job when you also have drain.