I can't think of any historical anecdotes, but my feeling and the feeling of a large number Reserves 2000 supporters goes like this: Let's focus on gender. If the unit requires, let's say, 20 new people in a recruiting year and you say, “Okay, half of those recruits have to be female,” and partway through the year, most likely towards the end of the year, you have only three, the chance of perhaps making that up with more males diminishes. Therefore, you can wind up at the end of the period missing the opportunity to enrol people who want to do it.
Also there is a danger that good candidates might be turned away. Let's go on that 50% thing again. If the places for 10 male recruits are all filled and there are a bunch of really good male candidates, and they've only been able to attract three females, the likelihood of good males being turned away is there.
Therefore, you could diminish the overall numbers and you could diminish the quality of an intake of recruits in any given year.