While customer satisfaction is a better (albeit murky) metric, value generated / profit is a better one ultimately. Of course, measuring developer productivity is a means to that end; how much did or will it cost to reach this value generated?

Anyway there's this adage that once a metric (like productivity) becomes a target it ceases to be a useful metric. But this doesn't seem to apply for value / revenue much, so I suppose it's good to keep an eye on this vague productivity metric.