Actually I'd say when marking English, it's more of a case of dropping marks. The examiner doesn't think, 'oh that's a nice sentence, I'll add a mark'. The easiest way to mark English is to rather see how much someone screws up and minus marks accordingly. The examiners don't even have criteria sheets and they mark the essays pretty quickly, so overall in their head, I'm pretty sure they'd opt for the method I just explained as it's much easier to do.
EDIT: Oh and I forgot to mention the main reason behind this. Good English is quite subjective so it's hard to add marks for 'good English', whereas bad English stands out pretty easily so it's much easier to just count the number of 'errors' and let everything else pass.