Because, each character has strengths and weaknesses. They each play different and are all fun in there own right. Playing each character lets me know there limitations and what situations each class shines. Playing just one character would be fine if that's what you have time for. But 3 of the same characters each between 30-32 is probably frustrating when you get gear for other characters. I personally have one of each character, i enjoy playing them all and dont really have a preference, so do the guys i play with. That way we can always be a well rounded fire team. Destiny already has minimal content to play and repeat and repeat. By having only 1 character type your literally repeating the exact same thing even more. Making the game even smaller and more repetitive. You can enjoy playing however you want, your just missing out on parts of the game.