I owe a lot of my interests in offbeat movies to two people. One, obviously, is Roger Ebert; the other is Danny Peary. The latter's Cult Movies books (which could really use a three-in-one reprint) talked lovingly but also sensibly about movies like Liquid Sky, Seconds, The Terminator and Faster, Pussycat! Kill! Kill! The last on that list opened with some words about "honesty" in filmmaking that may well have shaped my understanding of what people generally call trash cinema — movies that we know are not good for us, but we eat them up anyway.
Peary's argument, which is a little long to quote here, went something like this: Being honest about a trashy movie isn't much of a defense when you're using it as a dodge to avoid genuine criticism about a film. Alexandre Aja's Piranha 3D is currently basking in that kind of attention: sure, it's not great, but it makes no attempt to be anything other than a splatter film with poke-your-eye-out effects. Just as, to quote an example from my own catalog, Doomsday is not a great movie or even a particularly good one, but an awesome ride.
I go back and forth about this approach, because it can be problematic. The other day I watched an Australian flick named The Horseman, a grimy small-budget production that tried to merge art-house road-picture connections-between-the-generations filmmaking with a blood-spattered torture/revenge picture. The protagonist goes after the men who allegedly killed his daughter during the making of a porn movie. One of the tortures he metes out involves fishhooks and the one part of the male anatomy you would least want a fishhook to come near. This wasn't a "guilty pleasure" movie; it was too closely observed (and way too intimately, horribly violent) to qualify. That made it all the easier to criticize for falling short — far easier to attack than Piranha, for instance, a movie which allegedly seeks to do nothing but bring the goods. Most of the Angel Guts movies were pretty reprehensible, even if they were well-made and packed a wallop — there was a line, and they existed mainly on the other side of it.
Movies, and entertainments generally, all exist on different planes. They have to: they're created for different audiences, with different intentions, and with different outcomes. The movies we call guilty pleasures are going to be that much easier for us to defend because we enjoyed them, and not because there's some magical abstract quality about them. I find it easier to mount a defense for a movie I enjoyed, but at this point I know full well that's why. It's not because I have better taste than someone who attacks the same film.