Why not take Y Combinator public like Warren Buffet's Berkshire Hathaway? If you did, everybody could invest in the next big tech hits indirectly. Also, by investing in YC everybody could enjoy the diversified risk versus investing in much more volatile individual companies.
Have you considered taking Y Combinator public? If not, what are the compelling reasons to avoid that course of action?
Likewise, the only reason a firm should go public is if (1) they need capital, (2) they want a "currency" for acquisitions, or (3) they fell the firm is so overvalued by the public that it makes sense to sell shares. The first scenario is likely false for YC, the second wouldn't make sense either, and the third is exactly what we have above.
Here's my response when a similar question to this one was asked last month:
http://news.ycombinator.com/item?id=3893952