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I think the answer is going to depend on the artist and presentation. Some people use (or overuse) a sound--others own it. The listener is also a factor, of course.

James Taylor and his famous Revox A-77 open-reel tape deck come to mind. He has always had backup vocals as a part of his sound. But there are two completely different things. One is the tight harmonies that he creates with his own voice, uses for some of his album recordings, and still sometimes uses as a novelty on stage. It is obviously contrived, but completely accepted. The other is the beautiful ensemble sound he gets with small vocal ensembles. Both have a place and no one questions their validity. He has taken each and made them an integral part of his sound.

But you are talking about processed sound. A great example is Phil Collins. We have all heard chorus used to death on vocals and guitars. Mr. Collins has taken it and made it his own. Most people don't even recognize it as an effect--they just know instantly when Phil is singing.

I think it comes down to taste and creativity. Robot sounds probably aren't appropriate in a lot of pop music except as a novelty, or if an arrangement has been created in which they make sense. To me it comes down to the credibility of the artist. The average listener can't distinguish good MIDI instruments from the real thing. If a piece is properly mixed and skilfully performed I don't think that listener is going to be any more critical about the vocals. He/she may realize that they have a characteristic sound, but not be able to identify it any further.

On the other hand, if you're talking about a fractious bunch of, shall we say, persnickety alter kackers such as are sometimes found on music software fora, all bets are off. It's one reason I don't present more of my music here. I make music for my and my audiences' pleasure, not to be hypercritically deconstructed by jaded pros. I have fun making it and my fans seem to like it. To the detractors I say, if they can't take a joke, freak 'em.
</RANT>

"My primary musical instrument is the personal computer."