Does that mean that everyone who does that (and there seem to be a lot of people doing that) will shorten the life of her or his Mac? How?
It won't, typically, DDR3 based systems can address memory modules in banks of 1, 2 or 3. If a system supports it, installing memory in matched sets of 2 or 3 modules (dual channel or triple channel modes) will deliver greatly increased memory performance over running a single memory module by itself.