First you have many startup ventures that have limited growth due to the lack of cash, the $150k is a really good booster to achieve relatively high performance for most of the startups in this tech industry. The fact that this is a no cap/discount deal is unheard, which why it is bringing so much publicity. This is a game changer for the industry.
It might be a risky investment, but the odds are not bad since the investment is diversified into 43 talented ventures instead of one. But furthermore I think this investment is more than just money. YC and DST are investing in the future of the tech industry.
Don't think of this as an investment for the current YC portfolio, rather, think of it as a marketing strategy for future YC applicants. With this sort of publicity with DST, as well as connections with other Valley's giant investors (cough Sequoia cough) , all of the great talents across America will start to apply for Y-Combinator. I have a feeling that YC's going to find its Facebook real soon. It's just a matter of time.