I assume when you are talking about a bad power supply, you are talking about the internal connection. I worked as a computer technician for 2 years at a large retail chain (quit a few weeks ago), and I saw the same problem a lot.
I lost count of the number of laptops that had this problem. There is some internal connection that fails for two reasons. The first is physical damage/abuse to the power supply port on the laptop that comes from unplugging and plugging it in a bunch of times. After a while the wear and tear of this damagses the port to the point that it won't make a connection, and thus can't power the laptop. Sometimes this is obvious because the power pin is obviously bent or broken.
The second reason is some internal failure. Same symptoms, laptop won't accept ac power, thus rendering it useless, but there is no sign of damage. I have a toshiba laptop that is about 3.5 years old, and I have had this problem twice. The first was within the manufacturers warranty, and they fix it free of charge. In order to fix it, the tech notes showed that they replace the entire motherboard. This is a $700-800 part for laptops, depending on the laptop, plus another couple hundred for labor. It worked fine for another 2 years, and now I've been having the same problem happen again. I've backed up all my data, and just using it until it finally dies. It cost me around 2000 when I bought it, but I got my use out of it.
Thing is, I saw this problems mainly on HP's and Toshibas, but it happened to most brands. I'm not sure why so many laptops have it, it seems to be a design problem thats results in more frequent than average failure, but it isn't isolated to one brand or model. Just because several models came in during a small time period doesn't mean anything. I have seen this problem on many brands and models, and unfortunately there isn't anything you can do about it. Just buy an extended warranty on a laptop next time if you intend to use it for an extended period of time, laptops are one of the items it is actually worth it in my experience.