It's very hard to judge the relative size of various wedges, especially when they're not adjacent and one is significantly larger than the other.

Secondly, the labels are usually sufficiently removed from the data that you need to look back and forth several times to decode it, if you can even be bothered.

Here's a nearly useless pie chart: http://insights.chitika.com/uploads/screenshot-1.png

Here's a much more meaningful chart that tells a story: http://static.arstechnica.net/2012/04/02/firefox-adoption-20...