One thing you learn after being on NSC for a while, is that you can find someone, to say anything, claim it is true, and back it up with their own 'indisputable' evidence.
Seriously?
So you can use abbreviations, street talk and any other poor use of the English language, and if you fail your English GCSE appeal on the grounds of 'common usage'?
I don't mean that to sound flippant, but can you see where we are heading with that idea?
Were you ever taught it at school? Have you ever seen it written?
As we become 'lazier' speakers, abbreviations and short cuts creep into conversations, but that doesn't mean it's correct. 'Text speak' is a good example of communication that's all about brevity.