I can't think of any other reason why people would like Greninja then... it's design is mehish. And people actually do abuse Darkrai usage in Ubers, it's just out-sped by a Pokemon most think it should counter (Mewtwo). As for punching holes, it punches them only in choice situations and only if it's carrying Life Orb (shortening it's lifespan even more) And what power? 95/103 isn't powerful, it's just good. The only reason it seems powerful is from the STAB, but if you come in on a resisted attack or even a neutral one, it hardly does anything. I know this from experience because I've used it on Showdown and currently use it as part of one of my competitive teams on my game. It needs hazards for it to be a real threat to your opponent.
Chesnaught at least knows what it's job is, which is to wall the hell out of T-Tar (even Ice Beam sets) and hardly care what MegaKanga throws it's way. It's not gonna beat Special attackers, it's gonna die just as quick as Greninja does if it's in Talonflame's presence, and well, it's a pretty darn good Spikes user. Spikey Shield is one of the best Signature moves and easily the best between the 3 Kalos starters. As for Delphox, tutors are it's only chance, but if Gengar still can't get Sludge Wave through TM, Focus Blast just is not gonna in it's movepool. It could get the inferior Signal Beam though...which is something
That's opinion (and a bias one at that), personally I like its design. As for the Darkrai thing, the point was competitive value=/=popularity as Heatran is far from the most popular legendary of its generation.
Anyway, OHKOing a Dragonite through Multiscale isn't powerful? 103 isn't that impressive on its own I'll admit, but getting STAB on anything is and that's the selling point. It's not meant to be a sweeper, it's something to come in after you've done some damage to the other team and clean up the mess with hard hits, and if it sees something it doesn't like it U-Turns out of there. On top of that, whether or not you like Life Orb, it makes its attacks very powerful to the point that even if it doesn't kill, it's leaving something of a dent in almost whatever it attacks aside from special walls. You are right Life Orb makes it even more fragile, but like you said, it doesn't survive much, better make the most out of KOing what it can with those STAB moves and leave those dents then U-turn out if something bad for it comes in. Even if you don't use a Life Orb, Expert Bert still boosts the power behind its attacks if you're going for abusing those SE hits.
I won't deny Greninja is overhyped, but to say it's simply not good is completely misguided. It definitely has a place in OU.
As for Spiky Shield I think you're overrating a bit, if a good player falls for it once, they'll try to avoid it from then on (or abuse its lowered success rate next turn), plus Chesnaught's Achilles Heel is Special attacks and I'd assume most players would try to abuse that weakness over Physical hits if it can be helped. Spiky Shield's better then Water Shuriken and Magical Flame, I'll agree to that, but I feel that there are better options for Chesnaught to run.
As for Delphox, it can get it next game, Klinklang got Wild Charge in B2W2 after missing it in BW. Having said that, hopefully tutors can give it some good moves to play around with a la Serperior back in Gen 5.