It's not so simple as you make it out to be. The Iranian government is a complex mix of democracy and theocracy. If enough mullahs and enough of the populace are turned against Khamenei and Ahmadinejad, Khamenei and Ahmadinejad will fall. The murder of Neda Agha-Soltan, and any murders that follow hers, will turn more mullahs and more common Iranians than a fixed election ever would. (Everyone thought the elections were fixed anyway. That's how Ahmadi got elected the first time--the reformists boycotted the election.)
I don't disagree with anything you said. But, if I understand you correctly, you don't think that demonstrably fixed elections would effect that complex mix at all. I think they would.
My simple rational is that you're going to have more support from the people and lower levels of the government if one can provide compelling evidence the elections were fixed. There's a difference between most people thinking the elections are fixed, and actually having evidence to support the conclusion. Such knowledge emboldens people.
