HS and Cleave are non-normalized attacks. Meaning the add a flat amount of damage no matter what weapon you were using. So one cleave with Slayer of the Lifeless is does almost the same damage as one cleave with BP (almost because Slayer has lower dps).
To reiterate: what you're seeing is an illusion. Assuming the same dps - cleaving twice with a 1.5 speed weapon will give you twice as much bonus damage as cleaving once with a 3.0 speed weapon. But, the 3.0 speed weapon has a higher swing damage than the 1.5 speed one (since it has to do twice as much damage in half as many swings to match dps =p).
http://www.tankspot.com/forums/f97/4...red-sword.html for reference.
It is true that if you don't have the rage to spend, you may as well go with the slower weapon, especially if it has more dps like BP versus Slayer. When you have more rage through, you need to increase your weapon speed so you can spend it faster (either via cleave or HS).
So no it's not a general rule to use slow weapons. Rather you use slow weapons in low rage situations and fast weapons in high rage situations.