I would say that if you are already vested in production and know that you are serious about production, that it is a good investment and it is capable of a much more dynamic range of bass sounds than an "808", whatever that means in today's world to any specific individual. What I mean by that is that just about anything with sub bass character, people tend to call "808".
I know there have been several times where I say to myself, "Damn, I really wish I knew someone who plays bass to get on this track to play a funky bassline" that I simply could not produce myself on a synthesizer for one reason or another.
I think that bass (and sub bass) drums, synth bass and bass guitar are all great tools to be utilized. That being said, I think it comes down to budget and the type of hiphop that you are producing that would discern whether or not a bass guitar would be a necessity. For soul, funk, and just about any other genre, absolutely. Boom-Bap hiphop, absolutely. I would say it would be of utmost benefit. As for the modern whatever-it's-called pop music-type "rap", no. To be honest there isn't really room for any skill or intellect in this genre. --just honesty here.
Of course, I forgot to mention that you could also just invest in a good/versatile VST instrument that utilizes samples of a real and talented bassist to backup your requirements without having to drop a lot of dough and investing a lot of time in learning all the techniques that might already be available in a dope sound library?