So a few of you stepped up to plate when I asked whether or not a bug out bag was a needed prep. The answer: a pretty clear yes, not surprising considering the audience!
I completely agree. A bug out bag, 72 hour kit, good out of dodge bag or whatever you want to call it is most certainly an important prep.
A few scenarios...
There are a number of day-to-day disasters that could send you running out of your house with only a few moments to grab something--house fires, chemical spills, industrial accidents, etc. This stuff happens with fairly surprising frequency.
If your house is burning down around you, or a nearby refinery/factory/crashed train is releasing toxic chemicals or threatening to blow, you don't have a lot of time to run around and load your BOV with camping gear, guns, important documents, etc. pulled from different parts of the house. You've got a few minutes to grab stuff and go.
After TSHTF, your bug out bag will be a lifesaver if you have to ditch your vehicle--a very likely proposition if you are not "bugging in" at home. Running out of gas, breakdown, jammed or otherwise impassible roads, attack from a hostile force--there are a number of reasons why you might need to leave your BOV behind and set out on foot. The armed forces see the wisdom of this--pilots pack survival/E&E kits with them--so it must not be an entirely foolish idea.
Even if you live full-time at your retreat, you may still need to bug out. For example, it's TEOTWAWKI and despite your best efforts, a horde of refugee thugs has learned of your retreat. It's stand and fight to the death or run and live to fight another day--you'd be happy you had your bag-o-gear handy.
Some people are certainly more likely to need a bug out bag more than others. College students (or those in similar circumstances) who live far from home and who have minimal storage space are one good example. "Bugging in" for more than a few days is nor really an option if you're living in dorms or student housing. Expats living in 3rd world or potentially at-risk countries are particularly vulnerable, as the DVM blog (mentioned in my initial post) discusses here. People who live near refineries, chemical plants, busy harbors, inner city areas etc. may also be especially vulnerable to a "grab and go" scenario.
As many of the commentators mentioned, at the bare minimum, a BOB keeps some of your essential gear organized and in one place. You know what's in it and where it is--there's no searching through closets and rubbermaid containers or forgetting important supplies when TSHTF or even for more ordinary circumstances--a camping trip or whatever. Don't underestimate organization--you may have some advanced warning of the need to evacuate--but so will everyone else. The longer you spend getting your stuff organized, packed up, battening down the hatches and hesitating, the further back you will be in the long line of vehicles trying to get out. If you can be loaded up and on the road moments after you get the word, you'll be that much further ahead of the masses.
Part of the layered approach to preps
Really, a bug out bag should just be one part of your layered approach to preps. It's not the be-all-end-all. You should have ample preps at home, bins of preps packed and organized for bugging out, supplies in your vehicles, caches at your retreat location, backup supply caches, a well rounded EDC, a get home bag, and a bug out bag. You want options, backups and redundancy, and your bug out bag is one part of that.
I think people do certainly tend to over-emphasize the need for an uber-equipped zombocolypse survival bag. They're more fun to put together than saving away some emergency cash or putting back food storage and water--both of which you're far more likely to need. Shopping for tactical backpacks, camping gear, etc. is just more fun than saving cash or buying bags of rice and flour.
Remember to keep your "probable disasters" in mind when prioritizing where you spend your limited prep dollars. You're typically much more likely to need cash, food storage, first aid supplies, stored fuel, and many more things before you need a BOB and associated gear. If you're hard-up for cash, you'll probably want to devote your money to those kinds of preps first. That doesn't mean you have to go without a bag--you can put together a decent BOB on the cheap fairly easily--much of it is stuff that you probably have lying around the house or can find at thrift or dollar stores. Make sure it's not too heavy when you're done! If TSHTF and things are so bad that you're on foot, you'll most likely need to stay fast and maneuverable, avoiding trouble, crossing rough terrain and getting away from dangerous areas. Good luck moving quickly with a 90 pound pack on your back!
I would definitely think carefully before investing big money in a bug out bag before having other preps squared away. Especially when it comes to some of the really high-end stuff--the "luxury" tactical stuff like Kifaru packs, Strider knives, etc. A $500 Kifaru pack does little to increase your overall preparedness level--an extra $500 in food, water, fuel, or cash does. Be careful--it may be tempting, but focus on expanding your capabilities and supplies, not getting the coolest new gear.
Conclusion
So yes, everyone should have a bug out bag. There are plenty of instances where such a grab and go bag could be very useful. However, for most of us, those scenarios rank lower on the probability scale than many other scenarios, which would require "bugging in" preps like cash savings, food, water and fuel. Especially if those bug-in preps are well organized and could be quickly loaded into your BOV with a little advanced warning. Bug out bags are fun to put together and refine, and because of that, they often end up distracting us from investing in more important but more boring preps. Some should certainly devote more of their resources to bug out bags than others--but don't go overboard!