I think the ability to have the visualizer run when the show is running is more of a troubleshooting tool rather than a luxury tool, for me. In the past, I have been working on getting my controllers hooked up and getting all the last cords plugged in and everything, and I have ran the show in xLights to see if the sequences look good, and I always thought "Man, something looks off, I wish I had my visualizer playing to compare it too."
So I don't think the visualizer would necessarily be used during a nightly show, but rather during set up for troubleshooting; at least it would for me anyway.