You pack that bag once. You never use it. You never train with that weight. Then one day out of the blue you'll need to carry all that weight on your back and in a hurry? Possibly at least maybe 5 miles? If not more?
I agree. That's why we've spent the last couple of years actively practicing just that. Taking our BOBs out for trial runs, both in terms of the contents and also the weight. Basically hiking and camping with those specific kits. Camping/hiking/backpacking have always been a part of our lives, but thinking of those activities in terms of the kids and in and under the added stress of some sort of a disaster, adds two elements that can only be conquered, IMO, by actually practicing. Kind of like CPR. You hope you never have to use it but you take the refresher course every year so you're ready if you do anyway.
I'm confident that I can hike comfortably with my BOB, that the contents are sound (much trial and error, and a little downsizing/adjusting every time,) and that our skill levels (as individuals) are in a state of constant improvement, to the point where we could get by with much less than is in our kits. We've been working this Spring and Summer at integrating the baby into those plans, which quite frankly, have changed drastically since she was born. Believe me, I am in no rush to have to take her anywhere on foot for any length of time. Bugging in is still option #1, but thinking about and preparing what we'd need for her is a big priority.
Going to a hotel or relatives house by Jeep is our most likely scenario, but I do foresee the potential for us (in our AO at least) to be on foot, or at least ending up on our undeveloped five acres for a while. I'm not a Doomsdayer, I don't have an INCH kit, and I don't expect to be living the book "The Road" ever, but I have bugged out for a manmade disaster and bugged in during some of Mother Nature's specials. Our AO has a high potential for both, and I just think that the more mobile our family is, and the more comfortable we are living out of our packs, the more adaptable we'll be in general, but especially if a bug-out situation does arise.