If you are expecting this deal to suddenly turn Iran to Switzerland, no way. But very much like Rafsanjani changing direction towards some "capitalist ideas" (for his own benefit), Khatami trying dialogue, and Rouhani going for the deal (for keeping the IRI alive), they all have positive consequences.
When MirHossein/Karroubi were running, I heard it over and over that "they are all the same, don't vote" and "it's just a trick to get us out and brag about it to US/Israel." But we did. So they could not just win the election, they had to hijack it. And when they did, the resulting reaction of people who had engaged and were robbed resulted in the world (and more importantly: ourselves) know that we are not alone. That the will of people is different. That there is more to this country than likes of Ahmadinejad. If we learned something in 2009 elections, it should be that even if we can't change the outcome we can make it darn hard for the others to go about with their wishes.
I have this remote relative that for the past ~35 years refuses to participate in anything. She firmly believes that Mullahs will finally disappear and THEN a democratic administration will suddenly form. It has been 35 years, and nothing has happened. The only external existing group are MEK (MKO,...) which are terribly outdated useless communist/Islamist/(ex-)terrorist mixes which honestly deserve some elderly care at this point.
Now, by engaging and somewhat supporting the better guys we MIGHT get some improvements. The very simple example is Ahmadinejad's approach which was throw everything in, and try to ignore the sanctions. For people like him it is possible. He lives in ~600 AD. For us, it is not possible. Even for conservative muslims it is not possible anymore to live with conditions of that age.
We chose Rouhani, and he used the momentum to convince the more conservative elements of the government to push for talks. I am sure people like Naghdi still wanted to continue their heroic "resistance." We took part in rejecting that. And maybe, just maybe, this trend of change will allow a more open community. Maybe the will to include international corporations will become a motivation to improve the judicial system (even by a small margin). And that will improve lives, even by a small margin.
In short (TL;DR): if we do nothing saying "nothing will change," there are enough people who will simply steer it towards their own way. Let's challenge them, there is hope that we can win.