Bringing the debate out of the comments

From Sabernomics:

Here are the year-to-year correlations for pitchers throwing back-to-back 100+ innings seasons from 1980–2006.

Metric  Correlation
Strikeout Rate 0.79
Walk Rate 0.64
WHIP  0.42
ERA  0.37

All measures are correlated, but the correlation is lower for the metrics that include fielder contributions. The season-to-season correlation between individuals pitchers’ WHIP and ERA are quite similar. Also, both metrics vary similarly: the average coefficient of variation (mean/standard deviation) for the pitchers in the sample is 2.46 for WHIP and 1.99 for ERA.

Here is a graph of ERA and WHIP by age for Roger Clemens on that using connected scatter plots and quadratic fit curves.

The metrics tend to move in concert (correlation = 0.9), and the small difference in quadratic fit seems to be explained by a few more-extreme deviations in WHIP.

Thus, if WHIP has any advantage over ERA, it is slight...

Yup.