I disagree. We have a minivan with a third row. The only things ever packed in that row are pillows and a blanket. When we switch drivers, someone goes back there and gets two solid hours of sleep. Serious Zzzzzs.
I would disagree too if I were you. Why would anyone do something they thought was unsafe, especially with your kids? Definitely switching drivers and gettings some Zzzzs is safer than one driver driving straight through. I'd safe the safest way would be to do a total of 10-12 hours driving a day switching off between drivers and then getting a good night's sleep in a motel if the drive was more than 12 hours.
Everything is relative. It typically takes one accident for people to think otherwise though and then they find a way to afford the flight. Odds are you'll make it safely to where you are going though. I don't want to sound like I am mischaracterizing driving as being on par with rock climbing without a rope or something.

I'm a reasonable person.
But now we're getting off on a tangent. I think the main conclusions or observations I've made from this great discussion are:
1) Everyone's decision regarding flying versus driving involves a different set of circumstances and values that can vary greatly. But we can say with almost certainty most people (not all) who drive (that live far from the park), do it to save money.
2) Most people who drive, don't put a monetary value on vehicle depreciation, increased/decreased risk, and the cost of lost time otherwise spent doing a more enjoyable activity, into their equation (assuming they even bother to run any numbers at all.) This is just an observation. I'm not saying (here) if that is good or bad. Just an observation.
3) For families of four or more on a tight budget, driving almost always represents a significant cost savings as a percentage of the entire trip cost (with #2 above being a siginficant contributor to this outcome - otherwise the savings may be negligible depending on the value placed on less tangible costs as mentioned in #2.)
4) For families not on a tight budget (which appears most of the posters here do NOT fall into this category) who highly value the convenience of flying (whether it be time saved, not having to hear their children whine, fear of highway mishaps or some other facor or combination of factors), unless they are a large family of 5 or 6 living where airfares are higher than average, or live within a relatively short drive of the park (say 8 hours or less?), driving typically does not provide a big enough cost savings to justify the additional travel time, especially in the case of an airfare sale in which case driving can be more expensive, all things considered, especially if there isn't a need for a rental car and the car they would have driven has fairly low mileage (less than 40,000 miles). There are exceptions such as visiting relatives and doing a lot of sightseeing along the way.
4) Although flying is much safer, the risks associated with driving can be reduced by limiting the number of consecutive hours you spend on the highway, switching drivers, resting in a motel, etc. yet some people are still comfortable attempting a marathon straight-thru drive and have done it multiple times with no problems.
One note. On #3 there may be an urge to challenge that conclusion. So let me be more specific. When I say not on a tight budget, I'm talking about those that typically stay and eat at the Disney Resorts or something of close to the same caliber and cost. If you stay and eat at the Disney Resorts and live 800 or more miles away and still drive to save money, I'd love to here your story.
I know most of you are probably tired of reading now. I'll take a break from the marathon long posts.