That’s a really good question, however, I don’t think that only practicing piano or any other instrument will make you a better composer, why? Because playing an instrument and composing are two completely different things. The best example is HZ. He basically can’t play the piano (compared to other composers), however, still pulls off amazing music. Playing an instrument, knowing the strengths and weaknesses is important, but the biggest advantage for composing music is starting to listen to good music, whatever good means for you, and trying to understand what’s going on. I believe that playing easy melodies and chords is totally enough, piano, guitar, drums. whatever. Composing is about creating, not playing. I think it would be more beneficial to play for 30 minutes a day, and 90 minutes for creating new ideas, etc. what I do sometimes is only listening to one instrument only, for example a marimba. I have to know what is possible with that instrument, and what’s not possible. Far more important as playing the marimba myself for hours. It’s just a decision of your priority, as you can’t be amazing at all the things in life, so you need to focus on what is important for you now. Even if I can’t play something myself on the piano, I don’t care, as there are much better pianists out there, that play every single day, it’s their job. My job is to create music that they can play and have fun performing. Yes, playing one or more instruments gives you more benefits, but doesn’t guarantee a better composition. These are just two different fields.
PS: A lot of composers in Hollywood can play instruments, but never tell that they play them so good and constant so they would make a concert. One guy who sticks out is Brian Tyler. He is just on another level.