That type of system reminds me of how Wrath/Defiance stacks used to work, with your Crit rate ramping up at higher stacks. I don't think that it was ever beneficial to sit on your stacks for the passive, though.

Haste can be even more tricky because it also determines what weaves you can get away with without clipping. So if your GCD length keeps changing with your level of 'Darkside', then you'd have to know individual Darkside breakpoints at which you can double weave based on your own gear stats and latency. The last time we had a discussion about speed buffs (i.e. even Darkside being an 'always on' speed buff, there was some objection to that. I think it could be fun if it was tied in to a burst window (i.e. like original Blood Weapon), even if only for aesthetics.