Something I haven't seen mentioned yet, that you should point out to your husband, is how carseats on the plane help tremendously with children's behavior. Kids are used to their carseats, and they know that being in a carseat means they're going to sit for a while. Travel is stressful for kids, even when it's fun, because so much is different than they're used to. By not using a carseat on the plane, kids get confused and are more likely to act out. When carseats are used, kids are more likely to act just the way they act in the car. And even if they don't, it's much easier to keep them strapped down if you have a carseat than if you just have the plane's seat belts.
Having traveled by plane with kids, I've found that hauling the carseats is worth it JUST for that aspect alone, without even factoring in the safety issues. Even if someone could guarantee me 100% that I could travel by plane and car for an entire vacation without getting in a wreck, I'd still haul the seats just to keep the kids in a better mood.