Basic skills are important because they strengthen your writing. I dunno how many times last year when I was looking through the stimulous package there were people with strong theoretical answers that simply failed to explain it in a more coherant way because they were lacking more basic english skills.
So while of course I can understand 'we lernt that in yr 10', you would have to assume that someone who often makes mistakes like that is probably going to write similarily in their assignment and have problems conveying their message as well as someone who has got the basics down pact.
The other issue is that as the HSC is a mass-marked exam, having it based on more basic principles rather than more complex theories would make it easier to mark and less of a mind-dump.