A large proportion of new cars on offer these days come with what are called "lifetime" fluid fills in the transmission. What this is held to mean is that you shouldn't need to change the fluid in the lifetime of the car, unless you've abused it. Leaving aside for a moment the issue that virtually everything we do to our cars is technically abuse, and should have us on the "severe duty" schedule for servicing, what exactly is the lifetime of the car?
Well, any mechanical system has a lifetime. It will, eventually, succumb to age, wear, corrosion, and so on. This is true whether one is talking about the hinge on a tin of Altoids or the engine of a Volvo, despite the expected lifetimes with proper care being vastly different (and that difference is currently undefined; we don't know the lifetime of a properly maintained Volvo engine, because nobody's managed to wear one out yet). The lifetime depends on the lives of the individual components, and their ease of replacement. A good example of this is a perennial debate among car enthusiasts: the question of timing belts versus timing chains.
Valve timing has been a live issue ever since the camshaft migrated from its former position in the block to the cylinder heads. When it was down in the block, you simply put in a gearchain with suitable ratios and you were golden; the cams pushed the pushrods, which rocked the rockers, and the valves did their job. Sadly, all that extra mass was inefficient, and limited the engine's speed. Nowadays, the standard is for the camshaft to live up above the valves, and push on them either directly or via rocker arms. The pushrods and their play have been eliminated. Unfortunately, this setup means you can't use a geartrain to drive the camshaft; it would be too heavy and too awkward. A new solution was needed.
The two most common options are a chain, not unlike a bicycle chain, or a toothed belt. Both have their advantages and disadvantages. The chain is more durable; a metal chain will last longer than a mostly-rubber belt. The chain is lubricated, which the belt can't be thanks to oil's deleritous effects on rubber. However, the rubber belt is quieter, and it's a lot easier to change. Both are perfectly capable of maintaining valve timing within acceptable precision right up until they fail.
And fail they will. It's a fact of life, a fundamental law of physics, that everything breaks. Within a mechanical context, you have enemies; motion is the enemy. Anything that moves will wear. The lighter it is, and the more it moves, the more it will wear. The more complex it is, the more it will wear. A timing chain is a hideously complex assembly of very small parts, moving at high speed with close tolerances. The rollers of the chain wear. The links can stretch or even break. The sprockets wear. And because the chain is so closely integrated into the functioning of the engine, it's a pain to replace. Frankly, it's highly unlikely to be economic to repair an engine in which the timing chain is worn to the point of problems. It's generally held that a timing chain, unlike a timing belt, will last the lifetime of the engine; but looked at this way, what that really means is that the lifetime of the timing chain determines the lifetime of the engine. Any wear is repairable, but there is always a decision to be made as to whether the expense of repair is excessive compared to the benefits arising from performing said repair.
In contrast to the timing chain, a timing belt has a definite intended lifespan and is then expected to be replaced. It's generally hung out on the front of the engine, which exposes it to the harsh conditions in the car's engine bay but which also makes access for replacement easy. There are more oil seals to maintain with a belt setup; and remember that oil will destroy the belt in short order; but the cost of the belt replacement is essentially trivial. In this design, we've accepted that failure will occur, and planned for it by making replacement easy, and setting a schedule of maintenance which will in almost all cases result in the weak part being replaced before it fails.
What we're seeing here are two different philosophies of design. The chain represents the effort to reduce the chance of failure, at the expense of making failure catastrophic should it happen. The belt represents acceptance of failure as inevitable, and planning to remain one step ahead of failure. Both of these viewpoints are valid; I can easily accept a person looking at the problem of valve timing and, by impeccable logic, reaching exactly the opposite conclusion from me, even though I also used logic. The difference lies in the relative importance of various factors to the person making the decision. My preference is for a timing belt, on the grounds that it can be repaired more easily; others prefer chains because they need repair less frequently. Both preferences are logically arrived at, and that's why this issue remains a perennial flame-war. Both sides are talking past each other, because what they consider their killer points are what the other side thinks of as unimportant.
Going back to the "lifetime" fluid fills in transmissions, it's now clear that what the manufacturer really means is that the fluid fill determines the lifetime of the transmission. Remember, it's mechanical; it will wear out, and eventually break. Whether you consider the expected service lifetime acceptable is your decision; but remember, next time the salesman touts the lifetime fluid fill, to ask him what the lifetime IS. You should have that information to base your decision on. Or, you can opt for a car that has scheduled fluid changes, and accept that you'll pay more in recurring fees for a transmission that has the potential to last longer.
It comes down to accepting a black box, or accepting a slightly more finicky device that you can tweak to your satisfaction. Some things work well as black boxes; an iPod is a masterful black box. Other things work better as openable boxes; I prefer to at least have the option to pick and choose the components in my desktop computer.