Certainly, I would agree that that would be the ideal; however, there are situations when sleeping apart avoids sleep depravity and is a necessary "evil". Don't assume a married couple that sleeps apart doesn't get together in the same bed at times. If you think about it logically and practically, getting a good night's sleep goes far in benefitting a marriage.
Indeed. There can be scenarios, such as a newborn infant that cries a lot at night where the husband has to go to work the next morning and couldn't function at work while sleep-deprived. I have an issue where if I'm awakened by anything after about 2 hours post falling asleep, I cannot get back to sleep again. Or if, say, the husband snores, and the wife can't get any sleep. Or one of the two tends to flop a lot in bed and keeps waking the other one up. Or any similar practical considerations. Sleeping arrangements have nothing to do with the marriage debt, and as you wrote, the sleep-deprived can get cranky and short-fused, which is not helpful to a marriage.