I'm curious what that actually accomplishes in non marketing terms. That just means someone else is getting less renewable electricity, right?
On the other hand, if there is no government mandate, or if customer demand for renewable energy exceeds that mandate, then it provides a profit incentive for the utility to install/purchase more renewable energy.
They stopped, publicly reasoning that the proceeds from selling certs could be used to build their own additional renewable generation. I suspect it was just leaving too much money on the table, though.
Of course, even a 100% renewable supplier needs to buy from the market to smooth out peaks in energy consumption for their customer base.
(disclaimer: i work for such a renewable energy supplier)