Why do we have to care about coinjoin/mixing services?
Because
your bitcoins are being traced by
arbitrary analysis companies as we speak, in order to find any taint on them which any cooperating exchange will then freeze as soon as you make a deposit, without you even knowing about said taint.
Do you think coin mixing is the answer to this problem? If you use mixers, then the exchanges will freeze your funds again because now your coins are coming from a mixer.