"//It's important to ensure that people don't feel put off from studying such subjects, as still appears to be the case today//
And what evidence do you have that this is the case? "
A quick scan around most of my mathematics and physics lectures and colleagues shows a disproportionate number of men as compared to women. That's just eyewitness evidence -- one sees the same in numerous proper studies. Why this is in't clear. It may well be that many women, say, just don't want to do these subjects for entirely honest reasons that have nothing to do with perceived discrimination issues.
As to "why [should a diverse workforce be equivalent to a skilled one]", I mean that since there is no direct link between your skills and your ethnic background, gender etc., then a workforce that is not diverse is likely to be excluding a sizeable proportion of potential applicants not based on how good they are but on who they are.
The constant reply to this tends to run along the lines "if a white middle-class man is better-placed for the job than some other candidate from a minority ethnic background, why shouldn't he get it?" The answer is that he should -- on the other hand, one would also expect that on occasions proportionate to the population build-up, the better candidate will be a woman, or of some different background, etc etc etc.
Workforces without diversity statistically equivalent to the population make-up reveal, then, either some hidden prejudices against a certain type of person independent of their suitability for the job, or some problem further back in the line, whereby such people just aren't putting themselves up for the job or aren't reaching the required standard. If it's due to not applying, then you want to ensure that this is not because they feel that doing so would mean risking discrimination. If it's because they aren't reaching the required standards, then it follows that you want to address this.
I don't want a situation where someone is getting a job because of who they are in the "token woman" sense, of course -- ie, positive discrimination. A workforce should be diverse because it naturally is so that talent and background are not linked.