We think of the Canadian dollar as a general equilibrium variable. I know that sounds like a technical thing, but what I mean is that virtually everything that is going on in the world economy or in our economy has some feedback effect on the currency. That is why it is never simplified in the way you describe.
Oil prices, say, go down, and that causes the Canadian dollar to go down. The Canadian dollar didn't go down by itself, so there are two things happening at the same time. We know that lower oil prices are unambiguously a negative for the Canadian economy. The decline in the Canadian dollar helps to cushion that blow, but in the end we still have a negative for the Canadian economy.
Usually, when people ask questions like that, they think, “Well, if the exchange rate moves all by itself, is that good or bad?”
It is always a double-edged sword because for somebody it's good, and for somebody else it's bad. It is best not to think of it that way. It is more about its usefulness as a thing to keep things moving where they belong and, as you say, markets decide that best.
When the dollar is on the weak side, it is promoting exports of companies for which that matters. There are some that have a lot of imported inputs, so it matters less. For those who are thinking of investing in capital equipment, maybe an imported machine, it would cost more. The lower dollar causes them to slow down that decision, which would be good for economic growth if they did it, while at the same time speeding up demand for their products, which is obviously good. For every company, it is different.
Then, of course, there are the households, because the price for your imports, whether it is a vacation outside of Canada or simply fruits and vegetables, varies according to the exchange rate.
This is an extremely complex question, and I hope you will forgive me for not giving you a simple bottom line.