I use both channels on my vintage SVT. It does allow a little more flexability in setting the tone, and you can get sounds you can't get just using one channel. I never loop from input to input though (ex ch 1 brilliant to ch 2 normal) I split the signal with a stomp box with two out puts.
I like to run my guitar distortion stomp boxes into channel 2 with the ultra high switched in. With the effects off, you get little more attack by having the first channel set a little flat, and then add the ultra high on the second channel. It does sound different then just using one channel with the ultra high on. Same thing with the ultra low, or using the Mid sweep on channel 1 combined with channel 2. It seems to growl more at a lower volume using both channels too, but it is still way loud...
I change the settings on the amp a lot though, and the main reason I will use both channels is to blend clean and effected sounds to preserve the bottom end.
Sometimes I'll use the stereo out on my Ric 4001 with the Ric-o-sound box to dial in the EQ for each pick up, and get a volume boost with them both turned on.