Coshy, of course it can't be relied on to be 100% accurate. Neither can Brittanica, Scientific American, or any newspaper or textbook you could pick.
A scientific study done comparing Wikipedia with Encyclopedia Brittanica found the two were very similar in accuracy. Brittanica had a very slight edge on # of errors per article, and Wikipedia edged out Brittanica in depth of coverage.
Also, studies have shown that statistically, wrong information or vandalism has a shelf life measured in the minutes if not seconds. There are exceptions, of course, especially in subjects that are obscure, but arguing that "because someone could potentially vandalize it, it's useless" is like saying "because a car has the potential to crash, nobody should ever drive it".
Regarding the 'you can't cite Wikipedia as a primary source', well der, you're not supposed to cite an encyclopedia as a primary source, paper or electronic. It's a tool to get you started on learning something, or to get you a quick answer. But a primary source is always going to be something else.