Considering the number of images we push through the internet, a "retina"-resolution JPEG is still a factor of ten smaller than a PNG. Since many mobile plans have data caps, and many more are still forced to drop to EDGE speeds due to spotty coverage, it does play a role. Those that argue that image size does not play a role have not seen now average surfers have little to no tolerance for delays. Even speeding up the load time from three seconds to under a second is very important.
A PNG is still used a lot, though, due to its support of alpha channels. But that means they tend to be used where they can be cached. It has more or less replaced the venerable GIF for those areas where a SVG cannot (yet) be used. The old rule of thumb of choosing JPEG for photographic images and PNG for more solid colours still applies.