Of course, you shouldn't just let it end there, though. However, there's a lot of usefulness in diagnosing yourself using Internet resources. You can get a good idea of the problems you have before you ever see a doctor, and will even get sound advice on whether a doctor's visit is even necessary. As with all matters, simply use common sense about it.
Medicine is a complex subject that requires more study than a simple Google search. That's not to say that a layperson can never understand it, but until that person gets all the facts (and there are a lot of them) they should refrain from diagnosing themselves until they've consulted with a medical professional.
It is not at all wise to use the Internet for a medical diagnosis, because there is much more information that goes into making a responsible medical diagnosis than a list of symptoms. An examination, for one. And questions that only a doctor or practitioner would know to ask. While looking at diagnoses on the internet might prove somewhat enlightening, it is in no way a substitute for a medical exam. In fact, a person might become so overwrought at a mistaken supposed online diagnosis that they avoid their doctor, causing more harm.
The internet can certainly help figure out medical issues that are going on, but it cannot be the end all and be all of a diagnosis. People need to be seen in person to get a true idea of what they're going through and how to treat it, a computer screen at this point cannot replace that.