[vernie]

Please name some of those use cases.

[munchor]

Before imgur started keeping .png's, I just couldn't use it to show other people what I'm currently doing in my computer.

I share screenshots all the time and it really needs to be done with .png's for the clear image and text.

[stronglikedan]

Displaying text clearly and without artifacts surrounding it.

[wmblaettler]

Preserving alpha transparency and depending on JPG compression used, preventing artifacts/lossiness