If you're eating yeast, yes. If you're drinking beer, you're not eating yeast, except if you're drinking an "ale on lees" style in which case you'll get a little. With the large majority of beers, the amount of protein is tiny, and mostly what you're getting is yeast byproduct (ethanol).

I suppose it's all in your definition of "good carb source". As with the yeast, the grain that goes into making beer isn't really there in the end product: the yeast consumes the fermentable sugars in the grain, spits out ethanol, and the rest literally gets tossed out with the garbage. Trace amounts of this and that go along for the ride, but you're kidding yourself if you pretend that drinking a glass of beer is anything like eatiing a cake of brewer's yeast and a slice of whole-grain bread.