Why would anyone keep a bean on a mixing service for more than a few minutes? Anything other than in - out - gone is nuts.
Kudos to OP for badgering.
There are privacy considerations. The longer you wait to withdraw, the less likely you are to be linked to your original deposit re: blockchain analysis.
Chipmixer has discussed this before:
When you withdraw a chip, you receive a copy of private key encrypted on the mixer's server. You are free to sweep it yourself, ask us to send it to your address or keep it on chip for a while. Only the last option keeps your funds vunerable to our dishonesty, but it also extends your privacy. If you sweep funds from chip in first 12h after your input, you receive same privacy as you would get from standard mixer. If you trust us (as you already did when you sent coins into mixer) and spend chip when you need it, then you achieve maximum privacy you can get.
Aside from that, I've known people who used darknet market accounts as wallets, too. I also made the same mistake with exchange accounts in my earlier days. In my experience, people like to store coins on whatever third party services they normally use (out of convenience) until it comes back to bite them once or twice. Then they learn their lesson.