For a while, the story was that there were three songs that Frank said he didn't want anyone to play after he died - Watermelon, Black Napkins, and Zoot Allures (I believe). So to hear Dweezil break it out gave me chills - I didn't think I'd ever hear it live again.
And I get what you're saying about Joe's Garage. Act I is a solid album beginning to end. Acts II & III (released as a double LP) tend to meander a bit. There are some definite high points ("Stick It Out", "Dong Work For Yuda", "Outside Now", "Watermelon..." - at least - for me), but also some points where you think, if this wasn't on here, it really wouldn't make a difference. I had Act I for several years before I ever got II & III, and it grabbed me way harder than those two.