Would you say it's "OK" for them to use some mixer that could taint their coins if all they want is mere privacy?
Well, first of all, the anonymity set of 'all UTXOs' is much larger than 'only the
clean ones', so by definition, not caring about taint gives you more privacy. Do note that not all 'tainters' have the same definition; which means to be 'really on the safe side', you need to take the intersection of all 'clean pools' and try to get a UTXO out of there. I believe it will prove pretty difficult. I mean, what's the utility if you get a 'clean coinbase utxo' which you can
only send back to Coinbase, because other exchanges have a different definition and according to them, it is tainted?
What would you say to them if they're coins are withheld by an exchange because of taint?
You just told me your friends care about privacy, but then you are telling me they use a centralized exchange. This doesn't add up. If they care about privacy, they are using a decentralized exchange which doesn't care about some arbitrary party's definition of taint.
I have deleted my original post, in reply to you. I don't want to be offensive, but get the real context of my original point. "My friends" are ordinary users who merely want to make their transactions sent to porn sites, and sex toy merchants to be untraceable from their wallets. They don't care about privacy as "privacy in an idealistic sense". They merely want privacy in a practical sense.