Small pots and pans can absolutely be used on top of the Flat Top for heating water or making sauces. We do not recommend using pots larger than 1 quart on the Flat Top.
We do not recommend trying to boil water directly on the standard Flat Top for two reasons. First, it is far less efficient than placing the pot directly on top of the burner and will take much longer to bring the water to a boil. Second, the large pot of water can act like a "heat sink" pulling a tremendous amount of heat from the Flat Top and possibly warping the steel temporarily.
Our PRO Series Flat Top is made from thicker 3/8" material and is much more tolerant of extreme temperature differences without warping so you can use larger pots to boil water without fear of warpage but it will still be less efficient than placing the pot directly on the burner as the stove must heat the steel on the Flat Top before reaching the water and can take longer to do so.
If you find you will be boiling pots of water often you may wish to consider using a small plug-in hot plate. These are an economical and convenient alternative available online and most department stores.