I think it all depends.
Don't put them on if you don't intend to play the game much. You won't really see the difference, and then the next buyer can go, "Dang, this game looks amazing!" and you'll get a bit more money for it.
If you play the heck out of your games though, like I do, put them on. If your reasoning is you don't need to put them on until they start to show wear but you play it a lot, your game WILL start to show wear, and if you're one of those people that goes, "Oh man, your shooter lane has a slight dent in it" or whatever, it's easier to take the Cliffy off and show it as perfect then it is to say, "I wish I had this on here from the start."
As for longevity, I've had a set on my JP for going on 6 years now I think, maybe even 7. Playfield shows no additional damage from when I bought it. Game has had more than 3500 plays. I had one Cliffy *immediately* break, which he replaced immediately. Since that point, I have not had the protectors bend, move, or damage the playfield. There is NO reason I can think of to not do this.