To put on significant muscle carbs help as do a calorie excess. You could add more fat or protein into the diet for calories but you wouldn't get the same insulin response that you could from adding carbs into the mix.

... and carbs are nice for increasing training stamina although I tend to abuse coffee.

The other thing is that the micronutrients provided by a broader selection of vegetables and fruit is almost certainly a good thing on a maintenance or bulking diet.