In my early years entering to music and learning to play guitar I just couldn't have enough, for some fellow guitar players friends and musicians this was very egocentric and ambitious, I remember clearly some people telling me: "sooo now you want to be like a Mozart or something right? tsk tsk, ridiculous" I remember those days, but, I'm always searching for evolution, and doing things in the way I like it, that is why I learn others instruments. It's not something in thinking to be just a "one-man band" is more deep than that, for me, it's more musical knowledge, to know the different sonic possibilities that an instrument could add to your music, however, now I see that almost every composer should have a certain piano skills, for example, piano (the instrument) is in EVERY song that I write, could it be a tender ballad or a crazy avant-garde heavy thing! just saying!
All this come to my mind because of my musical heroes, they all can write music, and use other instruments as well, , people like Steven Wilson, Neil Morse, Frank Zappa etc. and is not just "learning to play well others instruments" is knowing how to use it and what its functions are! so, a person like me, could not just stand there and play guitar only all his life!! don't get me wrong, I LOVE the guitar and playing it, but music is more than that you know...learning other instruments give you more musical freedom, be able to write a cool harmonium part, or a groovy bass line, without depending on others to do it, it's so great! be able to write a whole song playing and experimenting with instrumentation and arrangement is a part of music that is so damn exciting to me, simply as that!