I prefer to see an OMB that is using harmony, to stick with just a duet type harmony at the most, and the harmony voice mixed rather low, almost at a subliminal level.
AND, not to use it too often...it tends not to be so cool if it is on most of the time...it's supposed to enhance and support, rather than dominate.
For recording, the sky's the limit, but too many vocal parts "live" on stage seems bit overdone to me, and, as Donny says above, it's almost "Karaoke" like.
Now, this is basically my opinion only...others may love lots of harmonies and such.
Ian
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Yamaha Tyros4, Yamaha MS-60S Powered Monitors(2), Yamaha CS-01, Yamaha TQ-5, Yamaha PSR-S775.