The way I look at it, it's a guideline. If you want to prolong your battery as much as possible, try not to let it discharge deeply on a regular basis. It doesn't mean you can never let it drop to zero every now and then, but if a user habitually lets it drop to zero, the battery won't last as long. I agree that it's clearly not feasible for most average users to watch their battery level like a hawk and make sure it's always between 40% and 80%. But on the other hand, I'm sure you know very well that we constantly get questions about batteries that are pooping out within 1-2 years, and I think it's reasonable to say that using a battery hard (i.e., deep discharging frequently) is a significant factor.