I agree that piano is valuable for all musicians, even when it isn’t strictly required.
My daughter (9th grade) is a cellist and has also taken classes in composition in the past. She became more serious about composition this year, and is also taking some serious music theory classes (this is all at a pre-college conservatory program). The composition program doesn’t strictly require piano (unlike some others), but at the beginning of this year she became very frustrated with how her low level of piano skill is holding her back in both music theory and composition. She’s taking a group piano class, but I can see that we are going to need to invest in private piano lessons pretty soon. She already has private cello lessons (of course) and private composition lessons… it all adds up to lots of $$$ as all the musician parents on this thread already know…!!