I like the weapon modals, but I think they need to be re-jigged a bit (at least the hatchet, which is what I was using). The hatchet modal is supposed to debuff accuracy on an enemy at the expense of damage output. However, I can activate it, get a 12 second debuff on an enemy, immediately deactive it, still get the 12 second debuff, and also have my damage returned to normal. So, now I have the best of both worlds for basically 10 seconds. I would think the idea behind a modal is choosing between two options with different strengths and weaknesses and not getting all the strengths and no weakness.
Of course, I can also just not switch the modal on and off, but that isn't really my point.
In summary: the strengths/weaknesses of a modal should immediately change and have no trailing effect when it is turned on and off.