It is for ease of maufacturing. There is no inherent weakness in this process. Fabrique Nationale (FN) pioneered this process, it is the way Browning P35 (High Power) barrels are made. Accuracy and barrel life are not adversly affected. They are quality barrels that will more than withstand several lifetimes of shooting. This is most often mentioned as a weakness by compeitors, since it is easy to imply a perceived although realistically non existant weakness. When is the last time you heard of a Browning barrel failure? Of course, there will always be those who claim their Springfield barrel failed, but most are those who impoperly altered their barrels, or more than likely is a repeated "gun shop" fabricated story. Then again, people do love to believe glossy advertising.
Im with FLSI
the size and price of the stock at 3/4" to make the barrels from verses at least 1 3/8 would add up. they could actually be stronger in that the grain sturcture of the stock could run across the barrel instead of with, but blue barrels do not shear off as often as ss ones. I read on this board one guy had one fail but I have never ever seen one. I have seen many one piece barrels shear the foot off though. They often shoot out of the box into 1 1/2 at 25 yds