Pie charts are not a good choice to communicate a group of numbers, because humans can’t compare area as good as length. Even if comparing values is not important, I think there are layout and readability problems with pie charts.
There are two ways to add legends to a pie chart. One method is to list each color and the name of the variable. This design has poor readability. The reader has to move back and forth between the pie chart and legend to read the chart.
A better design is to label each section in chart directly, obviating the need for a chart. This is definitely better to read. But the layout can be unpredictable. If the data is skewed then a lot of labels might overlap. For interaction designers, we don’t always know the data before hand. So using this type of chart will make the design unpredictable.
I think a simple bar chart is more effective than pie chart. A horizontal bar chart can be treated like a table. The data can be presented in an sorted order, which makes reading a more pleasant experience. Only one color is needed in the chart, which reduces unnecessary contrast.
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