I have a few bones to pick about some of the things stated here. Feel free to counter, if you have a background in nutrition.
I've taken three nutrition classes, the third one taught by a board certified nutritionist (And I know I still don't know everything). Despite what people think vet students are not turned into Hill's zombies in vet school. We have never been forced to go to a lecture taught by a Hill's vet. Our nutrition class had nothing to do with hill's food. Yes we get food for cheap, they help fund some of our clubs to go places (NOT hill's conventions) but do not expect anything in return. We did get a free small animal clinical nutrition book from them, but it wasn't authored by them!
I used to think Hill's food was terrible myself. I thought I knew everything, that corn was bad, hill's was crap, etc. Until I got 'educated'. I adopted a kitten that had chronic diarrhea. After running every test imaginable on her it was narrowed down to allergies or idiopathic IBD. I bought super high quality food, all natural, grain free, touched from heaven, etc. I tried every protein source on the market. Did it work? NO! My vet told me a billion times to try W/D but I resisted. After buying almost all the food on the market I finally caved and fed W/D - she's been on it for almost 2 years and hasn't had liquid diarrhea since.
Vets do not get paid to sell Hill's food, nor do they get kickbacks. The only thing they get is the small amount of profit from buying the food at wholesale and selling it themselves. Oh, we do get those little can lids - that MUST be why they recommend hill's!
Secondly, Science Diet/Hills is not "crap food" or "junk" - this implies the food is terrible, that your animal will suffer or deteriorate from being fed it. That your animal could do better on table scraps. Wrong. Is it the best food? No. But it certainly isn't as abominable as people make it out to be. It has it's place. No two foods will work the same for every animal. Many factors go into what makes one food better than another for a specific animal.
Yes, I wholeheartedly agree that higher quality ingredients are better, I prefer not to feed food with by-products, and have my own anecdote (dogs) for feeding better food, and having less poop to pick up. But I like to "spoil" my animals. They would thrive just fine on other foods - my cats are thriving on Hill's W/D, I almost wish they would thrive a little less so I could get a respite from their nonsense.
Thirdly, corn is not a filler. Corn is mostly starch - and when broken down slightly (either to a ground corn product or corn meal product) IT IS DIGESTIBLE. A real filler, for a cat, would be something like hay, something made of cellulose, hemicellulose, pectin and lignin. I swore up an down against feeding food with corn in it to my chins, but have since learned the error of my ways. Corn is actually a pretty darn good protein source. And when balanced with soybean meal you get a very nice amino acid profile.
Trust me, if they really wanted to make pet food cheaper and put in a filler, the companies would choose something MUCH cheaper than corn!