Littlesister, could you be more combative and rude? No, I don't work in a pet store.
If you actually looked around the food forum, you'd find links like these that tell you what is in your food, but obviously you haven't looked at all, even at the stickies that are "stuck" so that people read them first, which all makes me a little hesitant to bother helping you.
Like Skryker said, brewer's yeast is a good source of vitamin B, but there are better ingredients out there to get vitamin B. Why give a by-product of processes for making beer and alcoholic beverages than a whole, healthy ingredient that has other benefits too? I mean, good foods like chicken, bananas, turkey, oatmeal, salmon and others have vitamin B also. Wouldn't you rather feed one of those than a by-product with nothing else in it?
And sure, it's not the worst ingredient out there (not as bad as feeding corn that they don't digest at all or chicken by-products that can include the 4-D animals- dead, diseased, dying, disabled, or even soybean meal that can cause bloat in large dogs) but it's not the best either. Same with brewer's rice. Why put a cheap, nutrition-stripped by-product in there when real, whole rice isn't that expensive to begin with?
Here's one important link:
The sticky I posted it in: http://www.pets.ca/forum/showthread.php?t=13203
Here's another sticky you should read:
And if you google '"Brewer's yeast" and by-products' you'll get some links like these (among hundreds of others):