When I was younger, I was terrible at learning different styles of music at the same time. I wouldn't even allow myself to listen to styles of music that I wasn't studying. Once I started learning jazz, I traded in most of my non-jazz CDs so I could buy more Miles Davis, Jim Hall, Pat Metheny, Louis Armstrong--whatever I felt like I must have to learn more about jazz. Initially, I believed the artists I admired did the same. I thought that if you put a microscope on one area of your playing for long enough you will become a master. I no longer think this is true. I don't believe this is how you become a great musician today, and I doubt it was the approach for the musicians I've studied.
In the last 50 years, music has become incredibly diverse. There have always been many styles of music, but as society becomes more globalized, so does culture. 100 years ago, the music played in America could more or less be traced to its origins. Over time, the lines between one style and another were blurred, and in some cases, ignored altogether. Today, if you want to be a truly complete musician, it's important to accept, understand, and embrace as many types of music as possible.
More to the point, every day I'm reminded of this by the music I listen to. Bill Frisell is one of the best known jazz guitarists today, and his catalog ranges from avant-garde to Americana. The Beatles, who I've been listening to more and more recently, started out as a British blues band, playing the music of Ray Charles, Elvis, and Chuck Berry, and over the course of a decade became an incredibly diverse band. Herbie Hancock is nearly untouchable in his musicianship, and I can't imagine a style of music he wouldn't play.
When it comes to my practice routine, I no longer believe diversity equals unfocused. Rather, practicing different types of music exercises different parts of my mind. Some days I focus on improvising over altered dominant chords, or maybe syncopated funk rhythms. Other days, I just play acoustic guitar in the key of G, all day. There are even days I don't want to touch the guitar and instead play harmonica or mandolin. As long as I'm trying to get better, I think I'm headed in the same direction.
Eventually, I hope an individual voice will emerge. It will be the sum of all its influences, and it will be more recognizable and original if those influences are diverse. Music is music is music. To be better musicians, it's all worth our time and attention.