In my guild we expect 75% attendance, and people regularly sign off. We thus keep a slightly larger roster and people are expected that in cases where we have too many people, they may be benched to facilitate the best set up there is.
We're 11/13 10H having formed shortly after 5.2 hit, but we almost didn't raid for the whole of july due to vacations etc. We're classed as semi hardcore with 4 nights a week of which I attend 3. When 5.4 hits we will do normal asap, and we will also run flex in order to grab trinkets and set bonuses and fill in any non hc slots from the previous tier. The flex raids might not be mandatory, but I'm 99% sure almost everyone will turn up for them because it's basically free loot.
Similarly, if we could do 10 man as many times as we wanted, most of us would likely spam that a few times until we had most of what we needed.