Okay, I think I am convinced that moral responsibility doesn't require the ability to do otherwise. I find the Frankfurt-type counterexamples compelling in this regard. And perhaps even independently of the counterexamples, I think it makes more sense to say that moral responsibility must depend on whether or not you endorse what you are doing, or like what you are doing, or identify with your action, etc. than on whether or not you could have done otherwise.
That said, however, I'll also say that I'm not convinced that moral responsibility is compatible with determinism. In fact, I think the two are incompatible. Determinism, I think, is less like a Frankfurt-style counterexample and more like Disneyland. Let me explain.
You know that Disneyland ride where you get into a car that's fixed to a track and then you ride around on the car? Well, every time I go there I always have to fight to get the seat with the steering wheel. (We all want to be the driver, don't we?) And occasionally I succeed, and I get to pretend like I'm driving the car. Of course, I'm not actually driving the car, and I realize this.
But now suppose that I don't know that the car is on a track, and in fact I think that I am controlling the car. I turn the steering wheel to the right when I come to a turn, and (what do you know?) the car goes to the right. I have no idea that I didn't have any effect on the direction that the car turned. It seems to me that this is what determinism would be like, if it were true. In this case, I think I'm not morally responsible for the fact that the car went to the right. Of course, I may be morally responsible for something -- like turning the wheel, or something like that. But not for the fact that the car went to the right.
Compare this to a case where I am actually driving a real car that's not on a track and I turn right, but unbeknownst to me there was a counterfactual intervener that would have manipulated my brain and made me turn right had I shown an inclination to turn left. As it happens, I turn right on my own and the counterfactual intervener need not intervene. In this case, I think I am morally responsible for the fact that the car went to the right.
It may be that what's worrying me is sourcehood. And I'll admit - it's the fact that determinism is an actual-sequence mechanism that bothers me, whereas a counterfactual intervener is just a alternative-sequence mechanism. So, the question for you source incompatibilists out there -- why is determinism more like Disneyland? What about its being an actual-sequence mechanism makes the difference? And for you compatibilists out there -- why isn't determinism like Disneyland?