I’ve never been fortunate enough to take maternity leave, but boy have I benefited from it. When I changed career in the late 1990s from film and tv production in Ireland to technology policy in the UK, my first two jobs started as maternity cover positions. I was hired by the Confederation of British Industry and then by The Law Society to be an Internet policy wonk at a time when that was such a new thing, there were maybe half a dozen jobs doing it in the UK and I was one of the first people who’d trained especially for it. (The LSE hadn’t heard of Internet policy either, tbh, but they bemusedly let me write a Masters dissertation on it, because, well, why not?)
But the point is I don’t think either organisation would have been as keen to take a punt on a career-changer like me in a permanent job, even though both subsequently offered me one. A 9 - 12 month interim position was ideal for all three parties; me getting an opportunity to help create a new field, the employer trying out someone new effectively for free (everyone seems to forget that in the UK statutory maternity leave is paid by the government), and the person whose job it was who could take their proper leave and then come back to work.
Of course, where it gets messy and unfair is when employers decide they like the shiny new person more and shunt the returning worker aside, or when they don’t bother to cover the position at all and expect the existing workers to pick up the slack. I’m not saying those things never happen, even in a system where we have decent-ish protections for working mothers, just pointing out one happy though unintended consequence of maternity leave.
UPDATE: Of course the reality for UK working mothers on their return is often far from rosy, with half of them saying their careers have utterly stalled since having children, and a quarter believing they are discriminated against: http://www.bbc.co.uk/news/education-23600465