[mdda]

But the value of YourCompany today is (for any 'big idea') something like P(small)*Size(idea).

The trick of accepting the dilution of YC is to make estimates of whether the idea could 7% become larger/better. Or (more easy to understand) whether the effect of YC on all the components of P(small) could accumulate to more than 7% of P(small).

If your probability of success increases as a function of your investors, for instance, YC easily earns its 7% just due to pure signalling effect. If you're looking to get better hires, then the YC seal-of-approval could easily elevate you into the 'A-Class' of hires that everyone insists makes the difference. Of course, there are a lot of other factors : but scoring a total of +7% on them overall is all that should determine whether to choose a place with YC (supposing that you've been accepted).

Just my 2c.