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.
Please, don't throw stupid statements like "diminish by stacking a single stat". Spell Speed has an increasing return.

Note: wannabe-smart people try to repeat the fact that crit rating -> crit% is linear and say it has linear returns.
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.