In addition to all of the truisms above, it is also true that keyboard and synth basses and bass samples are all essentially imitating real (acoustic or electric) basses, and those general play one note at a time coming from one space (direction). More importantly, we tend to experience bass frequencies in a way that it is hard for us to pinpoint location (as opposed to mid and high frequency content). So, if you're not listening on your headphones, your stereo bass will essentially be heard as mono as you move out farther away from the speakers (we're it's also easier to hear the bass frequencies in the first place due to the physics involved). So for all of those reasons and the tradition (from the vinyl era), most people do not pan the bass very far from center or record in stereo.
That being said, I always move it and the bass drum off-center at least a few clicks, especially on vocal tunes. My personal philosophy is that the lead instrument (in that case, vox) should be the thing commanding attention at the center. Everything else gets bumped (at least a bit) to some degree, L or R...
GJ