My understanding of it is that it doesn't extend quite as low as a bass, and its only other selling point is that, in terms of timbre, it has similar characteristics as those which we'd find in the cello. Do we NEED an instrument that bridges that gap? History says no, most likely because the cello is so versatile, and the bass usually plays a specific function within orchestral works. Could a bass violin combine their functions? Maybe. But then we've got one less voice to work with, and from the standpoint of an imaginary composer in an imaginary time and place, I'll take bass and cello assuming different roles over bass violin trying to do both at the same time any day.