I wouldn't eat something that looked like crap. Either in actual appearance or in a photograph that is trying to entice me to eat it. And I wouldn't stop and think, "gee, I'm sure the actual food looks better than that crappy photo. I'm sure the advertiser was just trying to save money on photography."
What started this line of thinking was a stop at my local gas station mini-mart for their surprisingly good 99 cent coffee. But hanging next to the coffee station was a photograph of a "Baffo's Pizza Roll." It should have read "Barf-o's." The photography was that bad. The excrement (only word that does it justice) spilling out of the roll not only did NOT entice me to buy one, it put me off eating anything for a while. I wish I could include the photo here, but they don't have a website and the photo on their facebook page is too small to make heads or tails of (maybe that's what's inside the pizza roll!)
You don't sell steak, you sell the sizzle. When you sell food it is all about appearance, smell, and taste appeal. If your advertising isn't appealing, your sales won't be either! Often, all people know about you is through your advertising. So if you're not going to do it right, it's probably better not to do it at all!
In looking for bad food photography I came across this article from CoEd magazine that illustrates my point. http://coedmagazine.com/2009/03/13/the-art-of-bad-restaurant-food-photography/