I don't think a person has to learn new instruments to know the instrument. Let me put it this way, there are musicians that compose for symphonies. That doesn't mean that person has to know how to play every instrument: trumpet, tuba, cello, violin, bass drum, triangle, etc. That person just needs to know how each instrument sounds, their ranges and limitations, and how they interact.
For most people in this forum, you don't have to play drums, guitar, nor keyboard, but it's good to have a sense of how they interact with you and vice versa to maintain rhythm and sonic space.
The one thing I have learned that is even more important than learning another instrument (if you don't care to), it's how to project sound in any given venue. You don't have to be a trained sound-engineer, but at least understand how to get the best end-result sound for your audience.
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As far as learning guitar, if you don't want to, that's fine. It certainly won't make you a worse musician unless you're purposely learning just to fail. At the very least you're learning how chords are formed on a fretboard.
It's easy for me to write these opinions, I guess, because I am a multi-instrumentalist that composes once in a while. If I do write drum parts, I just have to remember not to compose anything that's physically impossible for one drummer like hitting 5 cymbals, 2 toms, and 3 snares at the same time.
