The Custom Reports feature is good at mixing dimensions together (up to 3 levels of dimensions), but all the metrics available are mostly "Standard metrics" in Piwik and a custom report can only contain one of the 100+ metrics available by default in Piwik.
Custom Metrics are especially useful to process a "KPI" value for a given day or week or month. From user feedback:
An example of such a metric would be the ratio between successful vs. total searches or the monthly/weekly/daily active user count. We define active as done at least 1 search per week (a metric that could also change in the future).
Custom Metrics would let us create new custom metrics and generate a widget / report showing the value of the metric for the selected website and time period.
Ideally this would be into core, or in Custom Dimensions, but this may need to be implemented in the Custom Reports plugin if not possible otherwise?
We could move it to 4.1