I'll save the argument that PG-13 horror movies suck, because that's obvious. The director's previous R-rated movie, Trick R' Treat worked because it balanced horror and comedy without limitations. This project should not have moved forward if they had to compromise on the rating and budget for special effects. The gingerbread men? Ugh!
First of all, this movie was always going to be a PG-13 film, and if you look at the movies Michael Dougherty references in his interviews, like Gremlins and Poltergeist and The Dark Crystal, none of those are R-Rated movies. He was going for a horror-comedy that was akin to a dark fairy tale, a movie that little kids could watch but also a movie little kids shouldn't watch.
And as for the gingerbread men, I thought they looked fine, and there was really no convincing way they could've been pulled off with practical effects.
"IMDB: Where Intelligent Film Discussion Goes To Die" - Scott 'The Foywonder' Foy; Dreadcentral.com
For me, the general rule is - if you can't pull off a scene with convincing special effects, it should not be done at all. You mentioned Gremlins and Poltergeist- which I love. Those movies didn't need CGI, and they're still impressive to watch. By the way, I know they were rated PG, but I think the ratings were looser then? I find still find Poltergeist to be scary.
Krampus wasn't trying to be scary in the same way that Poltergeist was. I never quite get why Michael keeps referring to that one, I guess because of the little touches of black humor, like at the end when they roll the TV outside. For me this movie is closer to Gremlins and Dark Crystal, especially the latter.
But even if you didn't like the Gingerbread men, I mean, do they really detract from all the other effects in the movie to you? Because to me, Krampus and Der Klown alone are two of the best looking movie monsters in recent memory. The scene when Krampus comes out of the fireplace, I don't know what it is about it but I adore that scene, it's so haunting and reminds me of Legend when Darkness comes out of the mirror.
"IMDB: Where Intelligent Film Discussion Goes To Die" - Scott 'The Foywonder' Foy; Dreadcentral.com
Why exactly would this movie have needed to be rated R?
Let's look at what's needed for a movie to get an R. Harsh language, bloody violence, sex/nudity, and gristly images.
Krampus probably could have gotten away with some more violence and disturbing imagery, but IMO it's creepy enough as is. It certainly didn't need any harsh language or sex.