I think Robbie makes a great video. Nevertheless, I think that someone with a punchier voice and a higher range could make the vocalizer sound better.

Robbie has a nice voice, but high harmonies sound better than low harmonies, generally. Piano players play a lot more closed chords with their right hand. And the breathiness of his voice is more suitable for solo. It's a voice I would like to listen to, but not in harmony that much.

I think that, as Scott said, he would sound better using the harmonizer on phrases rather than the whole song.