Personally I wouldn't use the word "smooth" but I suppose thats a subjective statement. I do think he adds a lot to the music thats hard to qualify. What makes him unique is that Bobby doesn't really play rhythm so much as he plays fills. Its very interesting to do a search for clips that isolate Bob's parts in the dead shows. It's eye opening insofar as sometimes you can't even identify the songs with the vocals, leads, and percussion stripped away because he really isn't keeping rhythm much of the time. But once you hear him in this way you won't be able to listen to the shows without him standing out in your ears.
Aside from that he's just a damn trooper, a true road dog. I have seen it estimated that no-one has spend more time on stage in front of more folks in the history of rock n roll. Its probably true when you factor that the dead played so many shows a year and as soon as the tour's over, he's out on the road with his other projects or he's sitting in. At 75 I don't think he even knows what else to do with himself besides go on stage every night and rock n roll.
(Here's an entire show of Bob in isolation)
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