I strongly believe that if you treat an instrument like another instrument, e.g. you're playing the drums with the mindset of a guitarist or a singer, you're actually extending the capabilities of the instrument you're playing, and you’re able to listen and understand the song as a whole. Proficiency with other instruments is not a requirement - understanding playing styles and concepts is enough.
Listen to how Toe drummer Takashi Kashikura plays. He's weaving in, out and along the lines of the band’s signature dual guitar arpeggios (edit: ok, it's not "signature", Minus the Bear and Pele do this, but Toe does this almost exclusively). He's playing his drums as a guitarist, so the music sounds more melodic, more refined, and more subtle. Through that, he's extending the definition of being a drummer.
In one of his interviews, he was asked to explain how he was able to play the intricate patterns and subtle time changes in their songs. He replied that he doesn't focus on the technicality of the songs at all, but he rather listens to the other band members and the overall mood of each song. Kashikura and the rest of the band are so good in transmitting the mood of their songs that listeners would often share the same song interpretations, even if Toe’s songs don’t have lyrics at all.
Beyond knowing and playing other instruments, a musician must master the skill of synesthesia - applying one sensation to another sensation - to be a true musical genius. For example, Hiromi Uehara, another excellent musician, was trained by her piano teacher to abandon all the technical stuff and focus on the combination of her senses. When her teacher was asking her to play the color blue, she played moody music, or when she was asked to play red, she played a really dynamic line. She was trained to play music as if she was painting images.
The musicians of The Mars Volta also compose and play songs this way. In one interview, Omar Rodriguez-Lopez stated that they don't focus on the technical side of the song, and they'd just write and categorize parts through visuals. They would have terms like "that pirate part" or "that jungle part" instead of “play 7/4 over this bass staccato”.
Otherwise, if you're a drummer merely playing as a drummer, you'd end up too technical and too self-absorbed.
