I think there are two sides (actually more than two) to this coin.

I tend to agree that if you are trying to objectively separate and review the music, listening to a show via recording is the only way -- even a webcast could taint your take on the music. And this cuts both ways. I've been to shows where I clearly was overly critical and on second listen heard some amazing moments that I missed at the show.

There are also shows that being there permanently changed how I listen to the show (not just in the melt your face moment). I would use the Hampton 98 run as an example. I have some friends that are seasoned fans that weren't there that on listen to this day insist that it was an overrated run. As someone who was there to experience that run, even listening to a recording 14 years later I still think it was really f___ing good music in addition to being an amazing weekend long party. So who's right? No one. In the end, they're all just opinions, which is one of things that makes subjective experience like Phish music so great.