I'm a guitarist who plays a bit of bass, it helps when writing songs and its good fun but there's a big difference between a guitarist and a bassist in terms of mentallity and the role of the instruments.
Although they both might share similar techniques and the fingerings etc make it very easy to pick up bass after learning guitar they are two different instruments altogether in how they are employed within a song.
One of my pet hates is when you see a "guitarist" who is playing bass and they just don't realise when and where notes should be, its the most obvious thing in the world and it does my nut in.
As a guitarist I personnally view drums and bass to be the most important aspects of a song, along with vocals perhaps. My guitar is there to add a little flavour, distinguish between minors and minor 7ths and to fill a bit of space but all the energy should come from the drum and bass, bassists are hugely underestimated and as a result they don't get the praise they deserve leading to nobody playing the bloody instrument.
Another reason for not taking up, or giving up, the bass that is often mentioned is that it is too easy.
This always makes me smile, as these people dont seem to realise that it is the music they are playing that is easy, and possibly does not require any more than root notes, with the odd "five" thrown in now and then.