Every Ligurian cook I observed diluted pesto with starchy cooking water, usually in a ratio of three parts pesto to one part water. This was not just Genoese parsimony at work. Pesto is concentrated, for easy storage, and meant to be diluted. The cooking water helps to bind the sauce to the pasta and helps the cheese to melt and evenly coat the strands. Pesto is often extended, and mellowed, with a sourish fresh white cheese.
Ricotta can serve the same purpose.
Ligurians sometimes add tomatoes to the basil and garlic. Plotkin calls this pesto corto, and his recipe mixes half a pound of seeded and diced plum tomatoes with one cup of pesto, or roughly one part sauce and one part diced tomato flesh. My favorite of the pesto-pasta combinations we tasted in Liguria was pesto lasagne, a lush piling on of riches that really lets you taste the pesto. Plotkin gives two recipes, and pesto lasagne is certainly easier than most kinds. Simply spread pesto over leaves of boiled pasta, dotting layers with parmigiano-reggiano and ricotta or gorgonzola.