This is a concept I constantly find myself struggling with. I do believe students should master different forms of writing (argumentative, informative, etc.), but I also believe it's crucial to provide opportunities for students to write in a way that is familiar to them. Personally, I tend to care more about the message my student is trying to send, and less about spelling and grammar in the moment. I believe conventions are important, but can always be fixed later. The interpretation of a text better represents a students ability to interact with literacy in my opinion. I always tell my students "we can always make it pretty later, I care about your reaction to what you just read more right now."