Please, don't throw stupid statements like "diminish by stacking a single stat". Spell Speed has an increasing return.No one gives a ****. What matters is the value. Value in this case is DPS, because no one really cares what stat is giving them DPS -- they just want results. The marginal benefit in results does, in fact, diminish by stacking a single stat.
It just depends of the point of view. If your boss gives you twice an increase of 1K, should you consider the second as a "diminushing returns" just because you were earning more ? And about the stats in general, stacking one is usualy not optimal just because they are multiplicating each other. This has nothing to do with diminushing return, the product of 2 linear functions is bilinear, not linear. The optimization problem is quite simple with Lagrange.Note: wannabe-smart people try to repeat the fact that crit rating -> crit% is linear and say it has linear returns.