I think the idea of cap and trade is that we need to drasticly cut emissions. The problem is it is hard to come up with a fair way for requiring all of the major polluters to cut emissions. The hope with "cap and trade" it that the free market can be used to fine-tune the original political decisions. If one major emitter finds it is prohibitively expensive to cut emissions because they are actually increasing production, they can buy credits on the carbon market. The hope is that industries with little growth may be able to accelerate equipment upgrades with funds from those industries growing faster than equipment upgrades help.
Of course, until every major government implements strong carbon caps, the carbon market is a shell game. Currently here in Canada they are still talking about "intensity-based targets"; that is to say: carbon releases are allowed to increase as long as there is economic growth. There is also the issue of using hard-to-measure things as "carbon sinks" such as forests. If a framer has a small forest they are being paid credits for; are the expected to pay back all that money if the forest burns down, releasing a lot of carbon?