For a variety of reasons, I think it’s a good idea for musicians to make sure to listen to people who don’t play the same instruments they do. Not as a garnish on a diet that is three-quarters dominated by their instrument, either. This is not a piece of parsley sitting next to a steak, here.
I mean, people who play an instrument at at least an intermediate level should have a musical diet that consists mostly of people who do not play the same instrument. (And no, “I play XYZ guitar, and that guy over there plays PDQ, so it’s totally different,” is insufficient. I mean, get away from your instrument completely.) The whys are as follows:
1) After a certain point, the only reason to know every obscure, useless detail about your instrument’s repertoire is to impress people. The hell with that.
2) You need to get used to hearing other sounds. It will help you to expect other sounds out of your own instrument and hence go looking for them. This will help you to sound unlike other people while still giving you a nice benchmark for beauty and musicality.
3) You need to not measure yourself, your achievements, or your abilities by those around you. This will help you go in the direction you are meant to go in rather than trying to beat someone else who just happens to be going in the same direction or becoming down because someone else got there first.
4) It helps you avoid GAS. If no one else is playing related gear, you won’t care what the hell they are buying or using. Neither Jeff Schmidt nor Rachel Barton Pine are playing stuff I can compare to my own setup. I avoid GAS when it comes to Gabriela Montero because her instruments literally cost as much as a house in some markets. A good Bosendorfer will run you around $85,000 not to mention the care and upkeep of one of those things. That one quick fact will cure any pianist of GAS right quick.
5) It keeps you from becoming stupidly tribal. You won’t identify with a crowd so quickly, and hence you won’t try to please them more than to please your audience or yourself nor succumb to the natural primate instinct to subsume your individuality into a glob of people when your ambition should be to stand out from them.
There’s probably more reasons. In the end, it’s just healthy to get the hell out of the dungeon in which the only other inhabitants are people who are all just like you.
I’d go so far as to say that maybe three-quarters of the music you listen to and any people you hang with should be those who do not play your instrument of choice. One-quarter can be playing the same device you play, and you should aspire to push that fraction down if you can manage it. The people who slip in under the wire should be very, very special.