You brought up a good point that I hadn't thought of when I made my first reply: GIF's color limitations. Since it is commonly only used for a preview, the reduction of colors isn't usually a problem. Anyone using it for final output is likely aware of its limitations and can work around them. However, it would be nice to be able to output a full-color animation, with full alpha channel.
To this end, I believe animated PNG export support should be added to Spriter. Although APNG isn't officially supported by the original designers of PNG, and it almost definitely isn't supported by the version of libpng against which Spriter is linked, it has become a de facto standard, and is more widely used and supported than the official, and IMO overly complicated, "MNG" format. It is even supported by most browsers, either directly or through some type of extension. It would be fairly straightforward to add support, either by using a version of libpng that supports APNG, or with APNG-specific code patched into the GIF export functionality. For reference, here is the Mozilla APNG spec.
