I play guitar in an "originals" band (I also record the bass parts as I'm teaching our bass player) and bass in a covers band. Whilst I get a thrill out of performing stuff I've written, I'm under no pretension that we are going to "make it" in any real sense, but the satisfaction you get if people sing along or come up afterwards and say "I really liked that song" is immense.
In the covers band, I get to let my hair down and enjoy myself. We change tempos, rhythms and even structures in order to fit our own playing styles. As a bass player, you can really let your imagination flow (unless the song relies heavily on the exising bass riff). In many ways, I'm playing for my own pleasure, rather than the audience. Playing covers also forces you to learn more styles, techniques etc. that you can utilise in your own writing.
Both types of band are equally valid, and the most important thing is to be out there playing with other musicians rather than be a bedroom bassist.