As a bassist I normally pluck with my right hand fingers. However, I wanted to introduce some variation to my playing, so now use a pic for some songs. These plectrums offer a secure, wide, grip area and are solid enough to produce a strong tone from the bass.
I also use them with an eight string bass (i.e. 4 courses of two strings) and acoustic twelve string guitars. For me, a plectrum is essential in getting a good sound from both instruments. In the case of the acoustic twelve, the improvement in tone over a thinner pick is substantial!