Texture-less painting can seem dull compared to other effects. But plain, flat paint may be desirable in conveying a sense of mechanical restraint by limiting the degree of gesture expressed in the paint.
When paint is extremely thinned, heavy pigment particles can separate from the binder. The pigments may then nestle into the ridges of the underlying support, accentuating the surface texture of the support. This effect, if desired, can be manipulated by using different fabrics as supports.
If you dilute paint, you must be careful to retain enough binder to hold the pigment to the support once dry. (Test a dilute mixture by rubbing a dried sample to see how much color comes off with physical contact.) Thinning solvents are used to make extremely dilute washes of oil paint. You should limit your exposure to them and always wear a respirator. To achieve a thin wash of acrylic paint, add water or very thin medium to the color mixture.