You pretty much hit the nail directly on the head with that description, Cap'n. The Grateful Dead were truly about filling out the melodic and rhythmic accompaniment to their songs, with each member bringing a fairly complex element into the mix. Jackie's playing does not really fit with the style, though I think seeing him doing his own stuff with an acoustic guitar could be much much better.
I really like that version of Brokedown Palace with just JG and Phil in Phil's basement, so he really isn't all bad, just not a great fit in a big intense band like Lesh has going.
a whole new gameball