Modern standalone harmonizers have MIDI (which would allow your arranger to call up the preset you want per Performance), they even have the ability to just LISTEN to audio and derive harmony from it, and basically, go to the demo pages of any of them and be amazed! There isn't honestly, an arranger that comes CLOSE.

The Korg is the best available, and that is based on TC stuff from 4-5 years ago. A lot has happened in the meantime.

And standalones USED to be studio gear, sure, but have you messed much with VoiceLive and others along those lines? Designed from the ground up to be live tools for singers. There's a lot more foot control than there used to be... One of the things that is difficult to do with an arranger is vary the harmonies much during a song. Sure, you can pick out a preset, one voice above, or block full harmonies, etc., but if you listen to real harmony, it is often different all the way through the song. One part here, a block there, one above, one above/one below, unison in some sections...

Most of the standalones give you multiple footswitches, which can alter the harmony as you go along. Bringing harmony in and out as required, to the degree needed is the hallmark of good harmony. Listening to arranger demos with block harmony voicing from beginning to the end (and a nasty, phase-y sounding block at that) simply makes me cringe. My G70's harmonizer is one of the better ones available (no Korg, though) and I can't bring myself to use it...

Better NO harmony than bad harmony!