Secondly, with such strongly hostile customer service representatives the more you speak that's more ammo you're giving them to argue without cancelling the contract. If you say, 'customer service is poor' there's a high chance that CSR going to pull up records saying that "You called us X times with Y and Z complaints and we have responded to them within 48 hours. That's well within our metrics" or some bs like that.
Talking to hostile CSRs is akin to talking to the cops - don't speak more than you have to.
I can't do this anymore because I have a small, few-channels package and 14-inch analog TV, and cheaper internet packages are bundled with HDTV cable plans. But thanks to anti-bundling laws (and a general sentiment that makes even McDonald's sell their toys independently of lunch boxes), internet-only packages are not unrealistically expensive.
Edit: My point is although it is none of their business why he wants to cancel, it's probably less effort, and helps them out by saying "The customer service is poor" than declining and arguing with the agent who is just trying to do his job (and probably getting paid $10/hour).