As someone of You said: there are many different basses, 4,5,6,7,8,9,10, 12 strings, but 4,5 and 6 string basses are most popular. why? because that ones are the most universal. These with double or triple strings like some Dean, ESP itd, have a special sound (like this in metallica's "wherever I may roam"). these with many single strings are too, hmm, difficult to play "simple" music (like rock, metal, pop, itd).
the main, hmm, job of bass guitar in the band is beeing a bridge between drums and guitars, pianos itd. They have to play main melodic line of song. there are not many guitar players, who plays grooves, or walkings, 'cause their job is play melody by chords or solos. Naturally, sometimes bass plays chords and main melody wiht guitar or solo. In music there aren't rigor rules.
there is another problem. on bass guitar chords in 1st position are just bad sounds. it's, hmm, rumbling noise. it is typical for low notes.
don't forget, that Bass Guitar evolved from this
instrument (what's it's name in english?).
I hope that my post helped to understand bass guitar. if I made mistakes, please, correct me.
And yes, I play bass guitar.