I think Collins had to have Katniss say yes in order for her (and the reader) to realize that with President Coin in charge it would be no better than with President Snow. The simple fact that Coin suggested another Hunger Games as retribution for what the Capitol had done showed that she was the same kind of creature as snow. When Katniss said yes and felt the anger and hatred behind her desire to see Capitol children die, that was when she realized, and me as well, that Coin and District 12 were not the solution, but just another turn in the cycle of death and violence Panem was built upon.
I was definitely shocked, though, but afterward I felt like it made sense. It also put Katniss on a level playing field with the people who invented the Hunger Games in the first place 76 years earlier, because she understood the rage that made people think it was acceptable, and in turn, realized that innocent people dying for other's crimes is wrong no matter how angry you are.
And yes, Prim dying sucked!!