Personal preference. I have one b/c I don't have to be as careful when I clean the screen; i.e. whether a cleaner has ammonia or alcohol, both of which will destroy the oleophobic coating.
But if you don't have one, the odds of you scratching the screen are slim. Some don't like adding a plastic layer to their screen, I get that. So again, it's whatever makes you comfortable.
You can get a cheap screen on Amazon for a couple bucks. One thing I'm certain of is that there is no quality difference between a $2 protector and a $20. The more expensive ones can thinner or be easier to put on, but never had a visual issue with the cheap ones, and they don't leave residue behind either.