Most illnesses are at their most contagious before symptoms appear, so keeping a child off once symptoms appear does nothing to curb the spread of disease.
Also a lot of the stomach bugs that are doing the rounds come on very very suddenly, so the child could go off to nursery/school in the morning feeling fine, and by lunch time could be puking everywhere, thus infecting others, but you can't keep a child off on the off chance that they might get ill.
There is whooping cough doing the rounds at my DD's school just now, which is at it's most infectious 2 to 3 weeks before symptoms appear, so some children that have it just now, might get it next week or the week after, while being contagious just now!
It was my DD's school concert today, out of the whole school there were less than 50 children in today, the rest were off sick! These things happen.
Children get ill, germs get passed round and immune systems get built up!
And I think it is madness to say that working parents should plan 3 weeks annual leave to illness!!! Not a chance!!!