Motor oil may be better now than it's ever been, but oil breaks down over time. Is synthetic oil even better? You bet. With synthetic oil you may be able to go 10,000-15,000 miles (that's basically a year of driving for most people) between changes now, whereas 30 years ago, it was recommended to change your family wagons oil every 3,000 miles.
Unfortunately over time the molecular structure of the oil does change. Oil is made up of long-chain hydrocarbons that provide a barrier to the metal parts of your engine that are rubbing against each other thousands of times a minute. When those chains break apart, the oil doesn't provide that much needed cushion.
When heat and oxygen combine with the oil, this increases acidity in the oil, and if left long enough, it forms varnishes and sludge in the engine. Trust me, adding an occasional quart of new oil doesn't do anything to improve the old, broken-down, acidic oil that's still pumping it way through your engine block.
And while the filter will trap any large particles that the oil picks up in its circulation, it won't do anything to prevent the oil's molecular breakdown.
So if you have a car that you want to keep for a long time, you absolutely should change the oil at the recommended factory interval suggested in your owners manual.