We've been cosleeping since day one (kids are currently 6 1/2) and have gradually fallen into "bedtime" that works for our family. For us, that means around 10pm. It's been a bit later recently because the air quality here has been SO SO bad that our ourdoor play time has been non-existent, so the kids just aren't as tired. We did have earlier bedtimes for a while as the kids were giving up naps and got tired earlier, but that was also based on their tiredness cues, not on some requirement that kids go to bed early. That was actually really hard for our family, since it seriously cut into the time DH had with his kids and it prevented us from doing anything as a family in the evenings.
I completely agree that it's critical to get enough sleep (for children AND adults). Enough sleep improves our attitudes and behavior, keeps us healthier, and lets us enjoy life more. However, the "rules" for children's sleep are just like most of the hard-and-fast parenting rules we read. . . they'll work for some kids and not for others. My son has always needed about the amount of sleep those "rules" claim he needs; his twin sister has always needed about 25% less. While I need about 10% more than the "rules" for adults say - needless to say, my DD wears me out! And I'm the oddball out in my house for sleep pattern as well - I'm an "early to bed, early to rise" person, while DH and both kids are night owls. Right now (I'm pregnant w/ #3), you'll often find me in bed alone at 8:30 and DH and the kids join me sometime around 10ish.
A late bedtime works really well for our family, since it give us most of our family time. Since DH works during the week and I work on Saturdays, that leaves only one day a week for us to spend together. Without a late bedtime, we'd have barely any time together. And since I prefer to get up early, I get my "alone" time in the morning. The only negative is that DH and I don't have couple time every day (perhaps this is where the early bedtime "rule" came from, since adult time in our society seems to given priority over kid/family time). But we can make time for each other, and we're adults so our needs are much less immediate than the kids'. And since I'm usually beat in the evenings, any "couple" time we had wouldn't be very rewarding with a cranky Mommy!
I think a lot of families also move to an earlier bedtime because once kids start school, they have to get up early. We're homeschooling, so there's no need to force the kids to go to sleep before their bodies say it's time. And I almost never have to drag a sleepy child out of bed. I think a more socially acceptable routine would be really hard on my kids - they just aren't energetic and motivated in the morning. Most of our best learning occurs in the late afternoon or during the evening. My husband, who's own natural rhythm is also to sleep late in the morning, is able to adjust his sleep cycle to get up for work in the morning. Obviously, he'd prefer to sleep in and stay up 'till midnight, but it's easier for an adult to realize cause/effect and modify as needed!
BTW, almost all the families I know IRL where the kids are in bed early use less-gentle sleep solutions. Most of us who are die-hard family bedders all go to bed as a family. Not all, there are certainly some kids who simply need an earlier bedtime, but usually there's also a parent who's more than ready to go to bed with them