I am always torn on this subject. On my phones I have always added a screen protector because I knew that I would upgrade and wanted it in pristine condition. I dropped my 6 plus and it landed on a little tiny pebble which cracked the tempered glass screen protector. If I wouldn't have had that protector on it, the phone would have been ruined. Do I want to have to put a screen protector and a case on a beautiful phone? No, not really, but I am clumsy and I drop things. I'd rather have a slightly ugly protector on my tech gadgets than have broken gadgets.
As for the watch, I am still really undecided. I have the Sport so I know that it would scratch easily and I have already hit it on a door jam which would have scratched the case, but I have the CaseMate Naked Tough case on it so it didn't get scratched. I have the Zagg but tried to install one and failed terribly. I got a replacement for it but have yet to install because I am completely clumsy. Anyway, like I said, I am still really undecided. If I had the Watch and not the Sport it would be a no-brainer, there would be nothing on my watch. I have never worn a watch that was any more than maybe $20 or $30 bucks so of course I've never used any protection on it. However, I absolutely love this watch and would like for it to last as long as possible. If I have to make it look not quite as nice, then I guess so be it...