pg's response doesn't exactly answer the question. crowdfunding start-ups would involve all investors dealing with all the start-ups. by investing in YC, YC would manage the portfolio of companies on behalf of shareholders.

the reason for YC to take investors would be to inject new capital that could be invested in more start-ups. imagine semi-annual YC batches of 1000 or more new companies. or seed rounds of 1 million or more per start-up, which is actually becoming increasingly common already.