I wasn't really talking about the sound, but the patterns. It would be akin to using only a couple of drum patterns for a whole track.
I probably noticed because I've had to play bass for several jams over the years, and for my own recordings. I'm a guitar player, and playing bass seemed like it would be simple, but listening to bass players is like listening a guitar solo. If you play the same riff over and over and over again, it gets boring. Slight timing changes add to the feel. It also helps to complement or enhance the simple chord structure of a blues track.