Just my two bits, as a vocal major in college, they require us to have at least a passing proficiency in piano and we don't even play piano, we sing. (well, I learned and played piano before these classes, but that's unrelated.) That gives you an idea of how important piano can be. HOWEVER, a MIDI guitar is definitely an [expensive] option - it even allows for things that you couldn't do with a MIDI keyboard.
Learning how to play the piano has really improved my overall comprehension of music-making in terms of composition.
I stopped lessons a long time ago, but the essence of what I learned stayed with me. It helps when doing composing or editing on the piano roll in the DAW. It also helps when overdubbing improv bits.
I also used to mess around with guitar a bit, but just teaching myself. I can't really play it that well.
But messing around with it gave me ideas of what it's strengths are.
So yeah, I'd say it's worth it to learn some kind of instrument that can play chords.
I wouldnt say its worth "learning piano" but it is never a bad thing to learn as much as you can. Its one thing to be a drummer, but its another thing to understand how they work to apply them to a mix.
So Its not that important that you "master" piano or keyboards, but rather learn some basic music theory and start from there.
Theory is universal, what can be applied to a piano can be applied to a guitar, flute etc.
Start somewhere, but dont let anyone tell you you're hooped if you dont play keys.
Everything that helps to express yourself in a creative way is worth, and it's strictly subjective to your musical endeavors. Maybe you don't need to become a piano master, but having a good grasp of the basics is important. I'm a total self taught and sometimes I struggle when I have something in my mind and I can't translate it in notes and chords.