As a British Primary School teacher I would say that a proportion of each holiday is used in assessments, preparations and planning ... up to as much as one week either side of the summer vacation.
However it's true that the holiday times are longer than in other professions, I wouldn't deny that. And this is a particular contrast in the USA where the European standard of 25 days paid leave is unusual, I really don't know how you cope or manage to have any time away as a family at all!
I suppose it's partly in recognition that teachers cannot choose when they have holidays - though of course they do get the major cultural holidays off that others have to juggle holiday entitlements in order to have ...
In Britain, especially, the costs for package holidays and holiday travel are hugely inflated in school holiday time - this year we got to England on a ferry before the British schools broke up and it cost us £400, to get back on the same journey within school holiday time was £770.
The long summer holiday comes from the convenience of farmers, not teachers - which is why the timing of it varies between England and Scotland and between various different European countries according to the weather. There is a lot of debate now as to whether this is still necessary - it is rare for children to be fully employed on the farms during harvest time due to the Health and Safety regulations regarding children working and the fact that so many machines are now used.
However teachers do tend to struggle with stress and exhaustion issues and one of the arguements is that they need a number of weeks off, together, in order to recover strength and sanity for the next term ... because it is such an intensive line of work. (One can't always go to the toilet when one feels the need, unless there's another member of staff in the room and even so that would disturb the flow of the lesson, one can't 'stretch one's legs and visit the coffee machine' and the breaktimes, although regualar, are not always preserved as a break ... too often there is a supervision duty (I don't mean yard duty, as that can still be a break) or a meeting or some necessary preparation - though the better schools that I have worked in have made sure that the teachers have had 10 mins to use the toilet facilities and have a drink ... regardless of other demands)
I'm not sure about those arguments, though I have noticed that my body 'succumbs' to a cold or similar virus every single school holiday, as soon as I relax and have the opportunity to be 'off duty'. So I usually spend the first cuple of days of each holiday ill!
I also understand that some of the holiday is 'in lieu' of the the extra hours worked during the week ... but that is because teachers, along with doctors and some others, were found to be exempt from the '40 working hours a week' European Law and found to work an average of 50 to 60 hours a week in an ordinary term-time week .... At one school I was certainly in school from 8am and 8pm 4 days a week and 8am to 6.30pm on the Friday only because I was running a Scout meeting at 7pm! I wasn't unusual, though some teachers do their marking and preparation in school - as I prefer - still in the working environment and with large tables to lay display work out on! - and some do their marking at home in front of the TV, which I have never managed unless it's Right/Wrong Maths or Spelling tests!
Hope that helps ... I'm not defending teachers on these issues, or saying that we should have a right to these wonderful holidays just because we are somehow more deserving than everyone else (!) ... but these are the arguments that I am aware of to support teacher's holidays.
Helen