wow, i thought i was being nice!
i think i know where we get off point from each other...you're looking at it from an ability/performance standpoint that you're better off learning how to play instruments that are (arguably) easier. it all greatly depends on the person, but i might be inclined to agree. guitar and drums were certainly easier for me, along with the 4 or 5 other instruments i can sort of play.
but for purposes of this discussion, i'm looking at it from a general all-around knowledge pov, and ability/performance is the least of my worries. and from that angle, it cannot be argued...piano is by far the best instrument to learn because of its visual aspect. not only can you hear the relationships between notes, but you can see them, and for that reason alone, it's got tremendous value. learn how to double on guitar, and you can hear the relationships, but every fret looks like every other so it's not intiutively visual. and piano is also a tremendous reference if you write music, even if you can't play it very well. you're only limited to the notes you can play at one time by the amount of fingers you have, so it's a lot easier to experiment with chords. guitar is great for chords as well, but it's got nothing on the keyboard because of its visual aspect. sorry, but that just plain can't be argued.
so if you want to learn instruments that you can perform on, learn whatever you want. but if you want to have a deeper understanding of chords and note relationships and an all around better understanding of how music works, piano > everything else.