PNGs are difficult to work with properly. Most software packages have lame PNG save support. Photoshop offers awful compression, to the point where getting a PNG as small as a GIF, or even close, is a very rare sight. I always wondered what the hype was over PNGs being smaller than GIFs. I almost never see that happen.
Gamma is a requirement, rather than an option, so PNGs show up in different brightnesses on every machine. I remember the early days of PNG where every time you saved your picture, it would keep getting darker and darker until it was black. You can only fix this by calibrating your machine, not setting a flag when saving your PNGs.
The only use I have for PNG is archiving pictures. For anything else, especially images for the web, I only use GIF of JPEG. GIF offers the compression I need for low-color images, and a high-quality JPEG looks pretty good (and small) next to a PNG.
There's just no need for it if you know beans about preparing web graphics. You can't blame IE for everything. I use what works best, and GIF and JPEG are what works for me.
What would be really helpful is to make a standard that offers functionality that is actually useful. Gamma is a start, but I'd like to see more formats with actual measurement and resolution settings, so I can define my documents in inches/centimeters, rather than pixels. JPEG offers this with metadata, which can be very unreliable with many software packages (such as the horrible Kodak DLS minilab I use at work). Anyone serious about press work uses TIFF files, which have no compression at all. I don't know how these people survive. :-o
