There is, and I'm currently writing up why.
It's mainly because they're so new nothing non-beta supports them, and with imageboards that can easily lead to users being able to sneakily post things that you can't see, because they're using Firefox 3.0 and you're using Safari or something.

The obvious abuse would be a two frame APNG, with acceptable content in frame 0 (the one non-APNG browsers show) and something nasty in 1, like child porn.