I'm a drummer who plays bass. I've been saying that for 15 years. Now it's becoming "I'm a drummer who plays bass and bad guitar". I'm teaching myself guitar because I want to be well-rounded.
I also want to be independent. For most of us, you can't play a solo gig on either bass or drums. You are pretty much going to be in a band. It's also _MUCH_ harder to sing and play bass than it is to sing and play guitar and I also think the guitar lends itself better to songwriting. Singing is important to me because it's so valuable and it will help you connect with a much larger range of people. Playing guitar and having that chord right there with your voice really helps you hone in in intonation. As someone who wants to write songs, keys or guitar are just shy of a necessary to truly internalize the sound of various chord voicings.
As somebody who doesn't want to be without options when the band breaks up and who would like the option to play solo gigs, learning guitar or keyboards is just shy of "a must". I like guitar because it's portable.
I don't really have aspirations to be a great guitarist. I know lots of really good guitarists and I'm not going to be able overtake them. If I need a guitarist, I'll ask. Too many other goals depend on me playing either guitars or keys. Right now it's guitar.
KO