Imagine the embarassement, going through stress or an accident or even drugs and forgetting it.
brainwallets are more of a gimmick. It makes better sense to keep em printed and offline.
What if you lost everything you own? What if you had to flee the country with just the clothes on your back? What if you did 10 years in prison and when you got out everything you owned was gone, even your email accounts closed for lack of use? Where would your bitcoins be then?
http://www.innocenceproject.org/Content/DNA_Exonerations_Nationwide.phpThink about this: If a prosecutor is so corrupt that he will get an innocent man sentenced to death, do you think he will have any qualms about getting an innocent person convicted of a drug crime or a sex crime?
I use to work as a research assistant. My job was to read criminal trial transcripts and make a list of all of the mistakes. The average trial has at least two or three major ones (that could affect actual guilt or length of sentence) and a dozen minor ones (probably don't affect guilt, but are a deeper sign of a corrupt system). You can appeal these issues, but they will almost always be ignored in state courts. You have to get to federal court for any hope of relief which takes about 10 years. The state courts know this and will take advantage of it if they know that you will be out within that time and just AWOP (Affirmed without Opinion) the case.
http://usnews.nbcnews.com/_news/2012/05/21/11756575-researchers-more-than-2000-false-convictions-in-past-23-years?liteThink about it. It takes a miracle to get a false conviction overturned. How often do miracles occur? 1 in a 100? 2000 known false convictions overturned would mean that there are 198,000 false convictions that weren't overturned. Besides, I think miracles happen less than one in a thousand. That would be close to two million false convictions in the last 23 years.
I can't prove it, but I believe that at least 10% of the people in prison are innocent of any crime. I also believe an additional 10% were not guilty of the crime that they were charged with (like being charged with robbery when only guilty of shoplifting) and given an unjustified sentence for their crime.
This is why brainwallets are so important.