Many students have approached me and asked whether it's ok to use informal language in free writing. I don't really have a definite answer to that, all I can say is it depends on the essay and how you structure it. Actually it's common to see informal language and formal language in texts.
One reason why- it adds realism to your text.
In Narratives, conversations and dialogues are always included. When we speak, we don't normally speak in formal language (we do, but not always). So in our writing, we use informal language to reflect what exactly took place. It makes the essay realistic.
Let me give you an example.
I was walking down the busy street when I caught sight of an ex-colleague, Mr Bank, at the other side of the road. I waved at him frantically and when he saw me at the corner of the street, he quickly ran over.
"Yo Stev-o! What's up man? You're looking dapper in your Armani suit and Prada shoes! Much better from the oversized tees and hanging denims you always wear!"
I was momentarily surprised. For someone who looks like he could not be bothered about clothes and fashion, Mr Banks sure knows his brands.
Here, the use of informal language in the conversation makes the story lively and interesting.
Let's try formalising it.
"Hello, Steve! How are you doing? You are looking very smart in your Armani suit and Prada shoes. It is much better compared to the loose and oversized t-shirts and denim pants I often see you wear."
