A few people have started down my path of thought, but haven't quite followed it as far as I do, so I'll toss in my two cents.
IMHO, we are discussing two separate things when we discuss the profession of agents/brokers in this thread: The profession itself, and the MLS.
I'll start first with the MLS. People have pointed out that its a local database, with exclusive access and information. Currently, the only real competitors to the MLS are websites that "scrape" data and compile it, ala Zillow, Trulia, etc. This is why they aren't able to ever match the speed and accuracy of the MLS, they're always depending on the MLS to provide the data first, and then only checking back every so often. With that model, everyone pointing out here that they will always be a step behind is correct, they absolutely will be. They don't control the information themselves, they just compile from what they can scrape.
The issue for NAR is that its highly unlikely that they will be able to maintain a monopoly on that data forever. With the money and clout that sites like Zillow now have, and the potential goldmine that would result from breaking NAR's monopoly on the data, all of a sudden you have a strong financial incentive for someone like Zillow to find a way to incentivize agents/brokers to list with them and not just the MLS, or perhaps instead of the MLS if access is made cheap/free/or even profitable. It hasn't happened yet, and may not happen tomorrow, but it will happen sooner than later. There is too much money involved for it not to happen.
The profession of agents/brokers, unlike the stock brokers and travel agents mentioned in other posts, is a value added profession as well as a gate keeper profession. By this I mean that most people can't effectively search for and purchase a home on their own without access to data, regulations, and contacts with title companies, escrow, etc. (the gate keeper portion) but also can't evaluate, negotiate and manage the functions of buying or selling a property (the value added portion). As I explained before, whenever there is strong financial incentive to break a monopoly on data, it usually happens, which means the gate keeper portion of the profession isn't something that agents/brokers can bank on forever. When the gate keeper professions of stock brokers and travel agents lost their stranglehold on the data, most didn't have value to add and thus disappeared. On the flip side, the value added portion of the profession is something that will likely never go away, simply because most buyers are not well read investors like those here on BP, most are buying a house with no intention of ever doing it again. Agents/brokers that make the process easy, understandable, quick and friendly will continue to earn excellent money and can charge a good commission.