Well, I'd say the notion of getting rich is self-interest-cognition-stimulating, but a productive business can certainly have a positive outcome for others, perhaps (due to scalability and feedback factors) even more than devoting time directly to charity. Generally, businesses provide more benefit (at least as subjectively perceived by the customer) than they receive. Kind of the point of a free market system.

Startups are indeed unlikely to succeed. But they don't cost (relatively) much to start, and generally fail within a few months. Compared to the gains if you are in one that succeeds, the risk is not all that high. Also, diligent Less Wrong students should be able to mitigate many of the risk factors better than average startup founders. ;)

Another huge benefit aside from entry into the getting rich lottery is education, i.e. from the school of hard knocks. According to PG, places like Yahoo would prefer a person who has started and failed at a startup over someone who has simply graduated college. Failure brings experience. And failing at a startup is not a smear on your work history like getting fired from another kind of job would be.

The business model that works best, according to Eric Reis, Steve Blank, etc., is to develop a customer base from the very beginning. Start with something with basic functionality and start upgrading it as you get more user feedback. If nobody will buy it in its rudimentary form, you might not have a very good idea to begin with.

Paul Graham suggests in the linked article that the importance of a good idea to the success of a startup is overrated.

Actually, startup ideas are not million dollar ideas, and here's an experiment you can try to prove it: just try to sell one. Nothing evolves faster than markets. The fact that there's no market for startup ideas suggests there's no demand. Which means, in the narrow sense of the word, that startup ideas are worthless.