I've had phones like a nexus 5 and 4, and tablet (nexus 7 2012) and not used a screen protector. I some of those devices had no scratches at all, some had micro scratches, others none. I've even dropped the Nexus 7 several time on tiled or wood floors, no case and cracked the bezel and still not screen scratches.
I've also had the Galaxy s3 and note 3 and had a screen protectors on them. It's a crap shoot to me.
I have one on my Note 4 because I'm worried about scratching it, but I'm not sure I really need it, but too afraid to take it off for now. In about 6 months, I'll probably take it off. I really think it's just a toss up and who you ask. Some people report they have scratches on their note 4s already, some don't...