If I mix my coins,, it will erase the history right? Like if I send my btc to an exchange, trade it to an alt and trade back to btc, withdraw back to a new address, then that coin will be clean, right? That is what I am doing, so I really do not care where my coins come from.
That's fine for preventing other people tracking your publicly associated addresses. Not much use if an exchange itself decides to get arsey with you.
My theoretical nightmare scenario is looking to cash out a bit in a few years by which point exchanges will be so uptight if they spot anything slightly questionable they'll tell you to bugger off completely. Since the vast majority of my coins are in addresses with a little bit of Bitmixer earnings they may decide I am fully tainted by the proceeds of evil.
If I myself mix them all before sending to an exchange they may still demand some sort of previous paper trail or flatly refuse to deal with mixed coins at all.
Hopefully they'll have long forgotten by then but as tracking gets better lord knows how many historical crimes will be linked to Bitmixer mixing.
I could well be excessively paranoid, but I wouldn't underestimate how heavy regulated channels may become.