I don't think the spell speed up is meant to be used tactically really, it's just a bonus, yes DoTs do crit and crit fester/painflare/deathflare yes please, so as a SMN you're going to be using crit regardless of if the spell speed buff is there or not, same goes for SCH, they use crit for the Aldo bonus so they're just going to have crit.

I'm not saying it couldn't benefit from a change or that a change to it wouldn't be nice, I'm sure there's plenty of more interesting things it could do, I'm just saying I don't think it was really designed with tactical use in mind and is just, there.