An exam too far?

The question has been raised as to whether GCSEs have any relevance any more. Their purpose was twofold - firstly as an end-of-school assessment for the benefit of prospective employers in respect of those leaving at 16, and secondly as a help to the school in classifying those progressing to sixth form.  But now pupils have to stay on at school until 18, what's the point?  Of course I'm sure schools would still want to carry out internal tests to evaluate how pupils are doing but abandoning formal external exams would surely save a lot of money.