A lot of people are bringing up moisture buildup. Moisture buildup/cold starts/not getting engine up to temp is a completely separate issue and is not related to oil change intervals. Moisture buildup can cause internal corrosion, but if you turn the car on from time to time it should lubricate well enough to prevent that. This is almost regardless of oil age or usage (just don't use 30k mile oil). Using old used up oil continuously creates sludge issues which are just as problematic.

Yes, you need to change your oil semi-regularly to prevent the breakdown of the oil. Think of your high school science classes - remember "half-life"? Similar concept here - oil additives break down over time, even if "unused". The oil viscosity also may be affected over time. This is a much smaller concern with synthetic oil, but it's still a concern. Oil does break down with time just as it does with usage, although on much different scales.

Now, the million dollar question is how long can you go. These bottles sit on the shelf for a while before being used, right? Well, here's the thing - there's no contact with internal engine components. Once you dump that quart into the engine, the additives start to interact with the metals in the block. Corrosion inhibitors are "absorbed" into the metal, etc. The chemical process has begun.

Realistically - you're probably fine for about 2 years before that oil is considered no longer "good", once that oil has been dumped into the engine. It will still work, but it won't lubricate or protect as effectively. If the oil had sat in the car for 3 years, even with only 40 mikes put on the clock, it's worth changing out just to get new corrosion inhibitor additives into the system, as they've probably been soaked up by the metal.

Now, is this a problem for granny's corolla? Probably not. But it may shorten the life of the engine for the next owner.