I was really interested in the New Vocaloid 4 as well as the CyberDiva sample voice pack, I have not bought them.
If you go listen to a lot of vocaloid music on youtube it becomes rather - fan made cheapy vocals. I did not find that the samples and examples around the web were enough to make me purchase the pricey Vocaloid 4 app/plugin and then a pricey sample voice pack (rough estimate is $300).
I think it would be cool or a good use as a backup voice or counter point to a main lead. but for long full length songs the sound of the robot/vocoder/cheap voice syth sound gets on my nerves and is taxing on the ears.
I think as a tool to provide trained singers how a song should go might be detrimental. I could hear a vocalist say "you want me to sing like THAT?!".