No it should not, bitcoin is perfectly fine as it is. Protip: You could always buy darkcoins, mix there, then move back into BTC.
The point is that if the protocol allows for coins not to be fungible (that is, to taint certain coins), and the axiom is trustlessness, then one cannot ask for a "gentleman's agreement" not to consider coins tainted. If fungible coins are demanded, it should be imposed by the protocol, and not rest upon the trust in an agreement. Otherwise, there's no point in having a cryptocurrency at all.