the problem is that we might have 'plans' for when we intend to start a family but they don't always go to plan. DH and I planned to start ttc at a certain point, then had to postpone because he was having a wobble about it, then in the end it took me over 2 years to get pregnant. I moved employer just a few months before I got pregnant, at which stage I had almost given up expecting it to ever happen or at least I expected we would need fertility treatment. So it is hard to see how an employer would hope to 'plan' for all this uncertainty.
also, I would be wary of it as my experience is that there are employers who pay enthusiastic lip service to the idea of being family friendly whilst harbouring numerous managers who are anything but (e.g.my colleague, a strong, kind and talented woman, was bullied ceaselessly throughout her pregnancy and again on her return from maternity leave, in a so called 'family friendly' organisation).
It worries me that if employers were allowed to ask, supposedly in the interests of 'planning' then they (or at least some ) would discriminate against a woman planning to start a family and yet in reality (due to changes in relationship circumstances/ infertility/ other) her plans might not materialise.
however, I do have some sympathy for the view in the sense that I was very open and upfront about having a child and wanting to balance work and childcare when I applied for a job whilst on maternity leave, and consequently I knew from day 1 (and have been proved right) that my employer was supportive of my need to balance the competing demands of work/ childcare: I have been able to work part time, I have adjusted my hours several times in the past 2 years for different reasons, there is no hassle whatsoever if my child is ill or I have childcare problems, I can always leave on time, its a pressured job but any extra work I do once my son is in bed and I claim back the hours (so can have bonus time with my son when things are calmer)