I only use screen protectors on my iPhones simply because, phones tend to suffer more abuse. They are kept in pockets, are exposed to elements more often (concrete floors) etc. I use my iPad at home and have a silicone back case and a Smart Keyboard which protects the screen. I don’t carry my iPad around outside and when using outside I tend to sit, or place the iPad on a table, which heavily reduces the risk of me dropping it. I simply think the risk of you scratching an iPad screen are significantly smaller than scratching an iPhone screen.