In a tank with mostly carpet plantings, no really bulky hardscape in it, the single lily pipe arrangement you described could provide enough circulation. But, the typical tank most of us end up with, has lots of plants growing to many different heights, and that tank, once the plants grow a lot, needs much more circulation. The mass of plants can create lots of dead areas in the water, with little or no circulation. You either need to be a very aggressive pruner, making sure the plant mass doesn't get too big, or you need additional powerhead(s) supplementing the circulation throughout the tank.