Ideally, the more 'in the middle' a battery is cycled, the longer it will last. Taking it to either the full or empty end will drastically shorten it's life, by as much as 50 percent or more in circumstances. So by giving you say a 18 kw battery, at it's Max, but letting you only cycle it 15 kw, gives basically around a 10 percent of a buffer at either end if set up properly. This could get a few more thousand cycles out of it over a complete cycle from top to bottom.
Running it fully up and down on occasion won't hurt it, and of course lets you see a general condition / capacity left on the thing, however over charging / discharging it will greatly kill it's life expectancy, even though the last few volts can kind of fly by fast, the damage is being done. Most battery BMS will cut a battery out when you get into danger territory but still, by the time they kick, damage is being done, much like by the time a fuse blows, something probably already BTFU before the protection kicked in.
Aaron