Previous versions of the iOS Simulator (7 and below) would also show the iPhone skin, which made for better demos:
Is there a way to do this with the iOS 8 Simulator? This isn't as compelling:
No, it's no longer possible. The recommended way for making demos is now using Quicktime in OS X Yosemite recorded directly from the device. If you wanted you could take that video and during post-production use one of the App Store Marketing Templates to make it look a little nicer.