I'd imagine in most cases it would be very disruptive to learning. While most schools will say that they plan to the individual children, fact is, the majority of work is taught to the class as a whole, with then additional allowances/effort/teaching to meet the needs of the individuals.
If you had kids dropping out any time during the year, it would mean the teacher who have to be tracking and repeating a lot of work.
Also, have you seen kids at the end of term? A lot of kids get ratty, you can tell when the term is a long one, you can see that some of the kids simply need a break.
By letting parents simply pick and choose, you are taking away that even spacing of breaks that currently exist. And what about parents who can't be bothered to sort out holidays.
Vacation care would be impossible, because it runs based on numbers, often in the school grounds. Working parents only get 4 weeks a year, what should they do the rest of the time?
Basically, not practical in my opinion.