But if you only play covers, then that is all there is to being a bassist, right?
Not really. Ever been in a cover band and the band leader goes to you, hey instead of playing it I-V-VI-IV lets start it on the IV and then move it up a step and a half for the verse? Or the band leader goes, hey I can't hit this chord tonight because my voice is shot, we'll do x instead.
Theory helps even in a cover band. Unless you're a really ****** cover band.