It goes against the web design principle of making your site usable to all users regardless of what they are browsing with to use an image format that is not widely supported. This principle is backed by users who will simply leave your site in disgust if they are asked to jump through hoops to use it. Very few designers will be willing to maintain two versions of all their images just because one of the formats is a bit smaller.
Microsoft has it's own range of newish web image formats that never caught on. History suggests IE is unlikely to support anything "not made here" unless it is forced to. The competition to Microsoft's image formats, JPEG2000, never caught on either. Firefox has an infamous open bug about adding it in dating back to 2000: https://bugzilla.mozilla.org/show_bug.cgi?id=36351 - over 11 years! Despite what it looks like in the last comment, my Firefox 8 still does not have support.
The last image format that managed to make it was PNG, and that was almost over a decade ago, although alpha support in PNG was added comparatively recently to IE. Last time I checked MNG (PNG's answer to animated GIFs) still wasn't widely supported.
Sorry to be such a downer, lack of support for JPEG2000 was a let down for me. As I see it Firefox, Opera and Safari will all need to support this first, before web designers in general will even consider using it. Only once a significant number of web designers are clamouring for it will IE slowly add support for it. Only then will it see wide use.
Tool support is vital too, most importantly Adobe PhotoShop. No web designer I know of is going to muck about in the commandline with imagemagick.