Interesting thread. Despite all the extensive quad-tracking talk I've read on here over the last couple years, I have never actually done it in the "standard" 100% L / 80%L /100% R / 80% R way. I've found a few places in songs with my band where it was nice to double two different guitar parts or something (and was definitely beneficial to the recording), but it always has just sounded like such a huge pain in the arse to full-on quad-track EVERYTHING. I've been able to achieve some pretty satisfying tones with just two takes! I think the next time around with my band, I'm definitely going to play around with quad tracking a little more, as I know I'm capable, but in the end, I don't know if it will really be worth all the hassle. "The End of Heartache" is still one of my favorite guitar tones ever, and I remember reading an article with Adam D. where he talks about how they "killed themselves" trying to get that tone (quad-tracking everything), and how he would never want to go through that again. I also know that on "As Daylight Dies" they did very little quad-tracking, and those tones are still pretty fatty and awesome too, so...
I would love to hear Andy's opinion in slightly greater detail on the subject, because I know he prefers to quad-track when there's enough time.