Cliffy's are great for:
1. Hiding existing and/or ugly damage. Especially for covering up wear around goble holes/ scoops that are frequently used. Prime examples of this include ST:TNG Neutral Zone, DE Jurassic Park, WhirlWind. If you look in the market section most of these machines have protectors in the scoops. You know looking at them that they will get blown out and adding protectors on these machines make them look nicer 100% of the time. I have a Cliffy on order for the "Stroke of Luck" scoop for my AFM for this reason.
2. Preventing shooter lane wear: The shooter lane protector is awesome. Mylar would work 90% as well, but IMO Cliffy's look better. Especially if used to hide existing damage.
3. Preserving high end machines that retain value. I have installed Cliffy's on only one of my machines, my MMr. I got it at under 1000 plays and intend to play the crap out of it. I trade games frequently. Installing Cliffy's on this machine will likely prevent a $ amount of damage equal to or greater than the cost of installing them when it is time to sell it.
Would I install Cliffy's in a random 80s or 90s B title to prevent wear? No. Would I do it to "spiff up" an area like a Whirlwind Scoop? Yes. Would I do it to a new machine that still looked perfect and might retain value? Yes.