I'm struggling to think of instances in which a pie chart represents data better than, say, a stacked bar chart. At least with a stacked bar chart, you can visually compare the heights of the constituent segments to see how the proportions have changed over time.

I have a book recommendation, by the way! Say It With Charts: The Executive's Guide to Visual Communication by Gene Zelazny. Granted, some bits are outdated for the types of data we look at these days but I found it useful. :)