I have been involved with mobile apps for eleven years. For those that are counting, that is pre-smartphone and pre-iPhone days. We launched a mobile coupon service in 2005, writing apps in Java, BREW, Symbian, Blackberry, and Windows Mobile. Since every phone was different in configuration, screen size, memory, and keypad, we typically had to write a different app for each phone. This required us to have the actual phone because emulators just were not sufficient for ensuring a good user experience on the phone. We ended up with around 200-300 phones and the phone room was a favorite place to take visitors. We also hoped for a future where mobile development would be simpler.
Flash forward to today - Facebook recently revealed their amazing mobile testing facility in Prineville, OR. Despite having only two major platforms to support, they have found that physical devices are the only way to ensure speed and responsiveness. Given the combination of devices and OS versions, they have around 2000 physical devices powered and connected at any given time.
So much for getting simpler.