Until Vidal starts outperforming Marchisio he deserves the bench. Pure and simple. Unless, like against Milan, Conte plays all 3 that's they way it should be.
Playing professional football means producing. This is exactly why every coach Juve has had has played Marchisio...consistently since 3 years now?
There is a reason he plays...there is a reason he starts...there is a reason why he had the top passing completion last year. Until a better player comes and SHOWS he can consistently produce more than Marchisio then we will continue to see him start. Sorry, but that's just the way it is. Its a business.
Like it or not, Marchisio offers assurances and a guaranteed hard effort and completion (consistently) of the basics, Conte is going to continue to choose him.