You should definitely avoid them if you have clean Bitcoins. There are two reasons for that. First is they are very expensive. You will get much less out than you put in. Second is you might end up with Bitcoins someone pay to hire a murder with.
What bitcoins do you consider clean? I mean if they weren't recieved through process of mining by collecting a block reward, then these bitcoin can't be called "clean" and even they may have history of transactions via CoinJoin or third-party mixers. Moreover, imagine the situation when there is no more newly mined bitcoins. People will have to deal with old bitcoins, which probably have been used in illegal activities many times. What can we do about that? Isn't it better to have an option to obfuscate linkability of coins in order to avoid a scenario in which all the coins in bitcoin network would be called illegal?