Now for all of you people who think english is "stupid" and just want to get back to your maths, phys and chem;
English improves your ability to write (spelling, punctuation, etc...), your writing speed, your ability to construct a logical arguement, familiarity with other text types (e.g. being able to write a letter in the proper format and very importantly how to write essays), your abilities in public speaking/debating/seech-writing, it gets you familiar with texts that are an important part of pop culture today hopefully inspiring you to further reading, and it also makes you question everything you read: to be aware of how people would criticise it, how the techniques are manipulating you, etc...
People who can't do all of these things will struggle greatly in later life. No, it absolutely is not necessary: especially as other subjects like history (With its essay writing and source evaluation) can do the exact same thing. But with English being compulsory it "gurantees" that most of society has these basic skills.
It is also fair that it is so hard on people that hate/can't read very well and foreign students. Because the UAI is almost the only thing universities look at for student applications, if english wasn't compuslory the VERY important fact that a student mightn't be able to write in syntax, speak english properly, or read a book or write an essay would not be factored in at all. Even though they might kick ass at other subjects, the fact english is universally relevant (essay writing, researching/criticising research books and english language skills being relevant to probably every uni course) means that they will actually be less competent than someone with slightly lower marks in other subjects, but who has actually passed english.
ENGLISH WINS.