Nicolas,
This is an old topic, but I just ran into this today. Many of our products have quantity discounts, but we have business partners that we allow to have steep discounts for just a single quantity. However, if they purchase enough for the quantity break and then apply the coupon, this results in a price that is too low.
On the coupon configuration page, there are options to "ignore discounted products" and "override discounted products". It seems like another option, "ignore quantity pricing" would make sense.
So I guess I am voting for such an option (or something similar).