My name is Jay. I play in a Modern Blues trio. I am the guitarist and singer. A lot of times, we'd play shows, and I'll hear a few fans, and even some musicians ask me, why just the three of us? Would it not be better for another guitarist, or perhaps a keyboard to add flavor? Most of the time I would give an excuse... We couldn't find one, etc. But over the course of our time together, I started to learn why I kept it "simple".
The top trio bands out there, Jimi Hendrix Experience, SRV & Double Trouble, Cream, all have a common thing. Their guitarists are PHENOMENAL. True. But one thing people don't see, is that the bassists, and the drummers, are also quite top notch themselves. The reason is a simple factor. Each person is essentially playing a dual-role. When everyone has to be at the top of their game, you can hear it. In a lot of bands, the more members you have, the looser things get. Obviously, it's not too noticeable, though it does happen. Another thing is your overall sound gets muddied. If you have a 5 piece band, two guitars, keys, etc. When they are all playing, who do you hear? Usually the singer... But do you know why? Because a wall of sound is behind them. Now this can be a great thing when done well. But with a trio, that wall becomes more like a window. You hear all elements, but on another level you hear more then what you would from a "full" band.
The reason behind this is purely psychology. Less is more, and in music, a lot of what people play isn't what you play, but what you don't. That can literally speak more volumes than the loudest screams, the blistering solos. Take for example solos. When you hear a solo on an album, you usually hear the rhythm guitar filling the role. But when you hear it live, unless they loop it, it's usually just straight solo, with the bass and drum keeping time. Notice then, that right there is when the magic happens. Why are Hexndrix, Vaughan, Mayer so good? Well, because they are, but they had the unique ability to harness "silence", which made their solos all the more epic.
Now don't think the trio has it easy. Without another guitarist, a missed note can really stand out. This is where the trio really starts to gain ground. The bassist and drummer form an almost singular sound. The drums will usually have a tight but driving beat, the bass won't just play what the guitarist does, but will add occasional trills and runs, eventually playing a song within itself. While all of this is happening you get the delivery of the vocals, and you have unision. All bands have that, but only Trios excel at it. Without it, the band just cannot work. Period. This brings in a comfort factor a lot of other bands don't have. You have to truly trust your band behind you. Do you think that Hendrix just chose Redding? No, it's because when he picked up bass, he had the ability to play rhythm guitar AND bass... On a bass! And Mitch Mitchell... The power, mixed with his off time fills, and the ability to play what the guitar does... On drums, is what gave him his sound.
I do not want to downplay anyone, but the story is this. The trio is an uncommon but powerful and intuitive group. Not just for blues. Green Day, Blink-182, Emerson, Lake, and Palmer, all of them had the bare essentials. And they stood out. With a mix of psychology, some good playing, and a comfort factor that cannot be matched, you can see why the Trio, while "odd," are one of the best musical decisions you can make.