I think GCSE's are important, mainly for those who aspire to carry on into further education and have a-levels to do first, but also for the 'workers' who don't have much to show prospective employers apart from their GCSE grades.
Even if you want to work at macky d's you'll be asked what grades you got, just the same as if you wanted to be Sir Alan Sugars new apprentice. hehe.
I think your parents are on the right track. Knowing that there's light at the end of the tunnel if you do yourself proud can only spur you on to great things.
Oh and Tula, I agree with you too that high grades aren't the be all and end all. A friend of mine got expelled from school 3 weeks before final exams and was destined to be a dole dosser, but he picked himself up, found himself a job and is now earning £500+ a week. I'm pretty sure he's the exception.