I don't get the benefit to working parents at all apart from possibly (but unlikely) cheaper foreign travel. DH & I both work and use a combination of our holidays and paid for holiday clubs/ childminders etc.
The number of holidays will be the same but surely by chopping them up into little bits it will become more difficult for things like holiday clubs/ activity clubs to function economically - e.g. how are they going to staff these things for a couple of weeks at a time at random dates - all different for different areas.
Plus I like the fact that kids have a nice long break between years to really wind down and refresh themselves. Plus it gives me time to do things like get all uniforms sorted out, get new kit for school and break a few of the bad habits that have crept in during the school year.
Inset days are bad enough when you have different children at different schools in different LEA's. But if different schools were able to pick different holidays you could end up even increasing the number of holiday days you had to provide child care for.