These plugins really exist to save time for large studios, not make bad musicians better. Time is money for studios, so they don't want to waste it on multiple retakes when someone can be close enough to make small fixes with melodyne. For session work, market effects still pressure people to, well, not make mistakes like that. A great singer is still going to be in higher demand than a decent one, because then the studios don't have to spend much time at all fixing their vocals.
Also, -noticeable- autotune can be desired. It's a musical choice. In that sense it's no different than using a vocoder, etc. I personally do not like it but that's the beauty of music; there's something for everyone.