In my opinion, a game should voice none or all of its characters.
Every voiced VN has the option to disable individual characters' voices, so those who prefer role-playing don't suffer whatsoever from an MC voice actor, whereas people who enjoy thinking of their MC's as individuals of their own like me lose out on the option not being there.
The only example I've experienced myself was "Rewrite"; the final act's giving Kotarou a voice was simply amazing, one of my favourite things about the entire game. Did it detract from the experience of those who had been role-playing as Kotarou? No, they could turn it off; meanwhile, in every other VN and game with non-voiced protags, I've had to imagine a voice for them, which is slightly awkward when I needn't do so for any other character.
Of course this isn't a problem in silent VN's or those silly 'Book' things, because rather than the most fundamental character with whom the reader spends most of his imaginary time being mysteriously mute and presumably speaking through telepathy or Sign Language to the rest of the cast, they're all reliant on the reader's imagination.