Why have a BUG? A BUG gives you a second chance if your primary is no longer useful. I think carrying a BUG makes more sense for those who intentionally place themselves at risk on a regular basis, but it is not a bad idea even for those of us who are risk averse, situationally aware, and who try to avoid dangerous situations whenever possible.
Is it possible to prepare for every contengency? No. You can, however, take basic steps that prepare you for the vast majority of situations you are likely to encounter in your daily life. I cook for groups as a side activity. I was an EMT for many years. I like to camp. I maintain first aid kits for the home, car, camping, and cooking, and even one for my greyhounds, and each is stocked according to the kinds of injuries I am likely to treat in those situations. We also maintain a disaster kit with several days worth of food and water. I've used every first aid kit. We completely used up our disaster kit during the aftermath of Hurricane Isabel, when we were without power for two weeks and under a boil order for several days. But we are prepared.
How does this apply to carry? We choose our weapons according to the threats we are likely to face and the environment in which we find ourselves each day. We can't prepare and equip ourselves for every possible scenario, but we can prepare and equip ourselves for the situations we are most likely to encounter. If we prepare well, we should be able to make due in those scenarios we couldn't foresee. I don't think carrying a BUG equates with trying to prepare for every possible situation. To me, carrying a BUG is like wearing suspenders along with your belt. If your belt doesn't reliably keep your pants up, then suspenders are a good thing. If you never have a problem keeping your pants up, then suspenders may be unnecessary, but they don't hurt. Sometimes, wearing suspenders along with your belt is awkward or doesn't make sense. Some people wear suspenders as a fashion statement. Some people wear suspenders as a matter of practicality or for their job. Most people don't wear suspenders.
Risk assessment. When I was in my 20's, I went through several years where I had to do mission assessments, remain situationally aware, and constantly reassess risk, daily. Old habits die hard. At the risk of sounding paranoid, I try to live my life situationally aware. I prefer to think of it as being prepared.