But they are fully known to you. The risk with mixing is that attitudes, techniques and surveillance will change over time. If staying put and doing nothing is a minimal or manageable risk then the path of least resistance makes most sense.
I think this should be threated case by case, as figmentofmyass said.
I prefer to keep some of my bitcoin anonymous. It depends on which country you live, what are your bitcoin sources and so on.
Just like attitudes,techniques and surveillance regarding mixers will change over time, those things may also change over time for bitcoin holders and people who receive bitcoin as payment.
I agree it is safer to "do nothing", but it is even safer to play both: do nothing with some coins and mix others.
There are risks when you don't mix them as well.
Read what I said, it's different for some users, and I believe it's more risky to mix coins if you're only doing Bitstamp - Your Wallet - Bitstamp/your other KYC/AML exchange.
Almost everyone who replied in this thread receive bitcoin as payment from casino or mixers.
Ofc if you only do bitstamp > wallet > bitstamp you have no reason to mix. Then you shouldn't mix. There are many risks involved, even the mixer service can run away with your money.