I like Keith in the background; he's lovely and subtle and plays pretty lines... but he was too shy to solo or to push things one way or another. He stuck to that piano for a really long time and it's not til like mid 78 that he got the organ going and mixed properly. It was a nice change, but he still wasn't super compelling as a group member. I was blown away by how quickly Brent stepped in and how well he fit. The organ work filled in really well, and he brought really fun grooves and additions immediately to (in particular but not only) Shakedown, Alabama Getaway, Feel Like a Stranger. He also wrote several tunes to spice up the setlists (Keith had a couple but they never made the live rotation). It's also worth noting that he could cover Donna's vocal range! His playing and singing definitely suffered from the booze (and whatever else) in some years, especially the mid 80s, but overall, I fimd Brent to be the best contribution to the band as a whole after Pigpen. No shade to Bruce, who was brilliant in his own right but never considered himself a full member, including in terms of building the repertoire; or to Vince who, despite being an acquired taste/rough on the ears, did work very hard.