Kids are tougher than we give them credit for, and by doing their homework for them (or practically doing it by "helping"), we are not letting them develop their problem solving skills, and we are basically setting them up for a fall. My 12 year old gets homework in the school holidays and I encourage it, they get a ridiculous amount of time off for holidays so they can easily accommodate some home work but still enjoy their holidays.
I think homework is a good tool to allow kids to become self motivated (ie they have to make themselves do the work), it encourages them to solve problems by themselves (provided the parents dont solve the problems for them), and it encourages a disciplined schedule. My daughter knows that when she gets home from school she has to get changed, get a drink, then start her homework, not only that but she has to do this without being told to do so. Once its done, the evening is hers, so there's still a balance of work and leisure time. It might seem a bit harsh, but these are skills that we need in adult life, and it does kids no harm to start developing these skills while they are still at school. I know some people have said that its a bit unfair while they are still at primary school, but the way I see it is this: if you allow kids to breeze through primary school, secondary school and all the homework, coursework and ultimately GCSE's will hit them like a ton of bricks, and this type of change to their schedule is what causes kids to suffer from stress. If you get them used to daily homework while they're young, they come to expect it and can handle the workload of secondary school much easier, because they're used to it. Just my opinion.