Evidence? I know free market theory sounds like the most plainly obvious, common sense set of statements but you still have to justify your conclusions. You are asserting without proof that the market is full of completely rational agents.
It's not that markets are completely rational at all, it comes from supply and demand. If you exclude a large portion of your supply the price will go up. For example, if I only higher people greater than 6 feet at my company, and my competitor doesn't care about height -- the wages will be lower, unless of course tall people are the cheapest to hire already.
I am not asserting that the market is full of completely rational agents. I am asserting that voluntarily excluding yourself from a resource pool or limiting your access to a resource pool will decrease your likelihood of success.
To give an analogy, if you have access to all the food you want, but you voluntarily limit yourself to kumquats and bananas for arbitrary, irrational reasons, in the long run you will develop health problems.
Likewise, a business that irrationally limits itself to a subset of all available talent will, in aggregate, lose out to those who are not thus limited.
I admit to not having citations of statistical evidence at hand, although I have seen many publications in the past that indicate diversity of employee populations is correlated with innovation and success. I did not provide evidence because I believe as a Gedankenexperiment, the conclusion is self-evident.
