Such an interesting concept that I still dont really understand. I keep my backpacking kit ready to go all the time , not in case of an emergency but because I hate having to scramble at the last minute to make sure I have everything ready to go. Every time I do last minute packing I forget something.
When I return from a trip now, I just replace things I used, wash clothes, care for the tools etc and repack. I have stuff sacks in the same closet clearly marked summer / winter and wet. They contain clothing and other misc needs for those weather types.
I dont guess it really constitutes a bug out bag but If If the wife and I grabbed our back packs and few stuff sacks we could survive indefinitely were it needed.
I still think knowledge and attitude is the most important thing to have ready all the time.
I dont understand the desire to go overboard with these things. You dont really need much to survive. At some point it is more about comfort. Having been through katrina even if you had a full kit ready to go, the highways were so clogged up you couldnt really get anywhere where you would get use out of the bag and the most important commodity quickly became fuel second to water. I have been through a few tornadoes as well, doing mop up after the fact as part of the FD. I guess some of those folks could have pitched a tent but when your house is gone and the community is in shambles there is usually a county or state / national support mechanism thats going to kick into gear and help.
So for me I dont know. I just dont really get the obsession some folks have with them, but hey its always good to think scenarios through. Yet there aint much you cant get through with a little knowledge and patience.
This is how I see it. I like the idea of having your often-used backpack ready to go...that's a bug out bag in my mind. Of course, you could add/subtract items based on your own worst-case/most-likely-case scenarios.
Some people don't do a lot of backpacking...and they should if they're considering a bug-out-bag. For most, it doesn't need to be a backpack...it could be a tuff-box or several large luggage bags especially if you plan to vacate by vehicle.
There are a whole host of scenarios that would make you have to vacate your home which Vec mentioned some. Wild fires, floods, earth quakes, chemical spill, major gas leak, hurricane. Most people would prefer to stay home and "bug-in", but there are some situations that could make that plan impossible...that's the need for the bug out bag. A 72+ hour kit that you could grab and go on short notice. Whether you stay at a motel, friends, relatives or even a camp ground, being even partially prepared will reduce a lot of the stress, especially for kids/pets.
PayetteRucker hit a very critical factor that is most often overlooked: A PLAN! Even in my job, we must have some "continuity of operations" for immediate base closure, local catastrophes or even regional catastrophes...the last means we may need to pack up and move to a subordinate unit on the West Coast.
There are a lot of logistics and planning required. You can brainstorm some of the most likely scenarios. For my family, it includes fire, chemical/bio spill (railroad is nearby), nuclear plan failure (located to our East), possible hurricane residue (from our SE/SW). Heck for the Masters, we bugged out to Florida initially and then returned home to do some backpacking

. Your plan needs to include primary/alternate destinations. You really need to have multiple routes.
Along with the typical "essentials" you need to cover, consider important papers, self defense (lethal/non-lethal), good communication (at least a mobile phone), and detailed maps (both locally and regionally).
For most people that don't do a lot of outdoors activities I recommend they plan to live out of a motel for 3-4 days, but I would add that they consider taking their own food and cook/prepare out of the room. It doesn't have to be the scenario of strapping on a mega pack with a full combat load and head off to some National Forest to hunker down for months...those types will last as long as their packed food and then they'll become refugees.
Figure out your most likely disaster scenarios for your region. Have a destination and develop a plan.
Lastly, I think most people should consider a "Bug-Home" bag. If you work even 10 miles from your home, it would be wise to have something to get you back home if you had to travel by foot.
ROCK6