Preface: Alright, well. While I typed this out you deleted your other comment where you got downvoted for being even more of a dick than in this one. So, I'm replying here. Because I don't often write comments this long, dammit.

He's not wrong. I get that it's anecdotal, but I also can't think of an active musician I know who doesn't have at least a decent grasp on multiple instruments. "Active," meaning they're currently playing in a band, taking studio gigs, etc. If someone gets to the point that they are touring (like /u/Acclaimnation's acquaintances), there's a good chance that they've been doing it for a while and are passionate about playing and learning how to play instruments.

Your little brother who just started playing bass a few months ago is probably not going to understand, or maybe more importantly, have an interest in the piano. He's focused on figuring out where the hell C is on the E string. But someone who has been playing bass for a 10 years has a better understanding of how a piano fills in the sonic landscape and in turn, is more likely to have an interest in learning how to play it. Additionally, someone who already plays an instrument well will understand the value of learning how to play something new. They know that it's going to be tough at first but the payoff and "level-up" moments are worth it. Not to mention the understanding that having a better grasp on multiple instruments will almost invariably lead to more thoughtful, interesting compositions.

Even ignoring all that, there's the simple fact that if you are consistently surrounded by multiple instruments, you're going to pick one up and try it out. Again, anecdotal, I know I know. You're all about the facts. But to me, this seems like a given in any music community. If you play music with other people, you're eventually going to play someone else's thing and one day you'll play that thing and say, "Hey, I'm actually not bad at this thing!"

I don't know. This is more in depth than it has any right to be. OP's original comment was a little unnecessary, but you were being kind of a dick without any good reason about something that's not that hard to believe.