There must be more to this comment. If you are not into vocals, then you can forget the harmonizer. But I doubt you meant to say something that obvious.

Do you think vocal harmony is not a useful effect? Or do you think that the harmonizers built into arrangers (or at least into a KN-6500) are not very good?

From Uncle Dave's review, no internal vocal harmonizer matches the quality of the VoicePrism external unit. I don't know if Scott Yee qualifies as being "into vocals", but he does earn his keep using the vocal harmonizer on his PSR-2000 to good effect--a fact that I have had the pleasure of confirming in person. I have listened to MP3s of several of the pros in this forum, and several use internal vocal harmony units. I particularly recall being impressed with an early posting by Don Mason.

I attribute a relative lack of interest in the Roland VA series to the lack of a vocal harmony option for those keyboards. The only forum member that I can think of at the moment who owns a Roland VA series synth is Andrea (Dreamer), and he is definitely not into vocals and has one of every other synth as well.

Of course, just as an arranger keyboard cannot really take the place of bandmates, a vocal harmonizer cannot really take the place of backup singers. But I bet among members of this forum, most of those into vocals do use a vocal harmony effect some of the time.