I can't fault anyone for having had enough and wanting to leave. I've been close, myself.
Here is why I think, in general, staying provides a better platform to affect change, for those who can manage it:
1 - the people I reach by staying are my acquaintances in my local ward. The people I can reach by leaving are Church leaders in SLC. The latter has already made the case that they alone drive change. Therefore, from their own point of view, they are not going to change a major point of "doctrine" or long-standing policy. People at the local level may not have any voice in the short-term, but by slowly changing elements of the culture, I believe there can be an indirect effect on a number of issues. I have already seen local people change on topics that the Church Leaders won't budge on.
2 - And this is going to be a parable. A local Priest wears an orange shirt when blessing the sacrament. The YML and BP both see him in the hallway after SM and remind him gently that he should wear white. Next week, he's wearing forest green. BP and YML now talk to the boy's dad. Next week, he's wearing a gold shirt. This time, he goes into the bishop's office with the BP and YML. They explain to the boy that he needs to wear a white shirt. The boy says that actually, that's not a requirement, maybe you should read the handbook again. The BP says, I understand, but in our ward, we wear white. If you can't wear a white shirt, I can't let you participate in the sacred ordinance. The boy leaves the office, starts a facebook group, gets a bunch of signatures, sends and email to the BP and YML stating that he is NOT going to wear white and he's talking to his friends too. Next Sunday, none of the priests have a white shirt. The BP and the YML bless the sacrament. The next week, most of the deacons are not wearing white. Escalation, threats of action, frustration arise. Eventually the boy resigns from the Church. From that moment forward, wearing of a non-white shirt is equated with faithlessness, being deceived by the Devil, disobedience and rebellion. Therefore, now all our boys will wear EXTRA-white shirts to keep them from becoming apostate, like that boy last year. My point is that when you resign over an issue, then that issue rises to a higher level of disobedience/apostasy in the view of the members of the Church.
3 - I don't think "us vs them" is a good way to win friends or promote meaningful discussion. We all the time complain about the Church's "us vs them" mentality, and then we do it ourselves sometimes.
I had collected my thoughts but On Own was more gracious and articulate.
The only thought I add, is that I have seen change come from within. It didn't change doctrine as much as practices. I watched my mom and other local stake leaders overturn the original block program plan of cancelling all midweek activities. When they first petitioned for the change they were met from all sides with "The Prophet has spoken ...." But they were gently persistent. In time the message was heard. Little by little change occurred. To be fair the new block program was still young enough that it was bendable, but the effort proves things can change from the bottom up.
Two piece garments were much the same way. The road a bit longer but the system was affected by long gentle efforts.
On most of the excommunication processes that have happened I have seen the black and white and not let it worry me. This effort I believe has a huge piece of caring for our flock behind it. I wish that the parties involved had both made changes in tactic because of the value of the issue.