Of course there is no Coinbase mixer. Yes, I just use their wallets for mixing. I don't play at online casinos. My coins come from normal, legal sources - mainly from exchanges. I have passed KYC long time ago and never had problems with Coinbase.
I don't understand why mixer would be more secure than Coinbase?
I wouldn't call it mixing since you withdraw tainted coins with your info atteched to them.
I wouldn't want any exchange to know more about me than I allow them to know. Whenever I buy coins from an exchange for long term holding purposes I run them through a mixer, simply because I don't want to lead them to my cold wallet stash, because if they know, what ensures me that a government can't get access to that information? Better safe than sorry.
In the end, it all comes down to who you want to protect yourself against. If you get coins from your neighbor but don't want him to know what you end up doing with them, all he will be able to know is that you sent the coins to Coinbase. I personally prefer that no entity but the mixer to knows (and that they delete logs as they promise) what the destination address(es) is/are.