explains the reasons for their transition from a relatively secure peer-peer architecture to a less secure client-server architecture. His comments take place in the context of a discussion of the NSA data gathering revelations on a popular discussion list.
Kaufman says the difficult, costly transition was driven by two things. The first was bugs in the most common peer operating system, Windows.
Second was the rise of phones, tablets and other portable devices. Desktop PCs are typically connected at high speed for long periods of time, but portable devices are not. When they are connected, it is over slow, expensive links and the protocol consumes a lot of power.
These considerations apply to any application, not just Skype.