Just to confirm you're anecdotal evidence, I work on a very busy Genius bar in an Apple Retail store and yes, there are many, many people who run apps straight from the disk image. Skype seems to be the most common app for this, probably because it is the most popular app that doesn't use an installer package. It does seem very easy to simply drag and drop an app into the app folder, and it's easy to scoff at people who can't/don't do so, but if you approach this from the point of view of someone who simply does not care 1tiny bit about even the smallest detail about how computers work, and has a busy life and needs to make a Skype call then it's easy to see how, once presented with the app icon this could come about.