The Joy of Cooking recommends serving beans, dried legumes in particular, together with carbs (tomatoes, rice) or other proteins. The reason: beans are a source of valuable, though incomplete, protein.
What do they mean by ‘incomplete?’
They mean there are one or more “essential” amino acids (amino acids your body needs that it can’t make itself, so it must get from food) that you can’t get from eating beans.
And they do not recommend serving beans with starch; they recommend serving beans with complementary proteins, such as those found in cereal grains. These are also a good source of carbohydrates, but that’s NOT why they’re recommended.
It’s not that beans completely lack any of the essential amino acids, but the amount of one (IIRC, lysine) is insufficient, and grains complements beans nicely since it has an abundance of that EAA and lacks itself sufficient quantities of another. Now, I’ve really forgot which ones they are, but a google search will disclose it. All that is important is that beans and grains eaten in the same day will provide you with all the EAA in sufficient quantities.
It was once thought that you had to eat them at the same meal. It is now known that this is not necessary. You just have to eat them the same day.
I forgot to mention that soybeans are the exception. They contain sufficient quantities of all of the EAA.