But what about taste and smell? Somehow the holodeck reproduces those--and in exactly the right places and at the right moments.
That I'd consider nuances irrelevant to the argument... It's just technology, and we have done much more wondrous things with technology already. It doesn't break any currently known laws of physics, either, like warp or transporters or phasers appear to do.
But Timo, the issue here is that some of the holocharacters become true, independent, self-aware sentients, not just facsimiles; which we know they are, because we're in the third-person omniscient position and can see what the characters can't, including the authors' tone, which makes it plain that these are true sentients and not Turing-passing substitutes.
Oh, I completely denounce the idea that Turing-pass would be distinguishable from sentience. Self-awareness is something you can Turing-fake just as well as any other property, and there's no way of telling whether something like that is fake or real, which IMHO simply means there's no distinction between the two in this case. We all simply Turing each other, and if a machine can do it, it's good enough to play a sentient holocharacter.
That is, a stupid Leisure Suit Larry program from 1986 is sentient until otherwise proven (and that will never happen) - it's just a very stupid sort of sentient. And mundane advances in computing will make Leisure Suit Larry XXVIII a pretty smart sort of sentient. It doesn't matter who is being Turinged: the flesh-and-blood co-characters, the audience, or the holographic sentience itself. It's still just a matter of appearances. And again, very mundane advances in technology will already give such good appearances that us poor humans never could tell the difference.