Some bands do just fine with very simple bass lines that are largely for rhythm. Other bands do very well with very complicated bass lines (see Rush for an extreme example of this). The good bands could be argued to be good probably in part because they are exceptions (thus exceptional). In general, though, bass players are often less melodically and harmonically inclined. That's why I really do like it when they step up. Drummers too. Dream Theater's now former drummer was an integral part of the team, musically and organizationally, and Rush's drummer is their primary lyricist. Again, however, these are exceptional examples.
On the other hand, there are a lot of people who pick up a guitar and play lead by only playing very simple melodies and who have very poor tonality. I wish these were exceptions, but alas...