If the effects of joining YCombinator increases your final valuation by more than 7%, then on a purely financial basis you should join YC because even though you have given them 7%, you end up more wealthy than you would have. Ignoring dilution.

Obviously there are a variety of caveats, such as voting issues and time investment.

Personally, I suspect most founders are at a negotiating disadvantage (knowledge, power and bargaining asymmetries), and think it would be easy to get an extra 7% by having YC on your side of the table.

Also see https://www.ycombinator.com/deal/ for details.