This sounds right to me, though there's a good counterargument: judges. Dobson might be pissed, but what he really cares about is judicial appointments, and he knows that even Giuliani will appoint judges that he likes. Hillary Clinton or Barack Obama, by contrast, certainly won't. So in the end, even if Rudy gets the GOP nomination, he'll swallow hard and endorse him.
It's important to distinguish between two cases here. One is that Giuliani is going to be looking for judges who'll uphold unlimited executive power, unrestrained corporate greed, and be unsympathetic to criminal defendants and that judges who hold their views are, statistically speaking, likely to issue more Dobson-friendly rulings on sex-and-death issues than are Clinton or Obama judges. I think that's clearly true, but key religious right leaders can't afford to be persuaded by it. A power broker needs to be seen to have power, and vague promises to appoint strict constructionists coming from a pro-choice, pro-gay, twice divorces lapsed catholic don't seem to me to demonstrate any particular clout on the part of religious right leaders.
Now Rudy has flirted with something more drastic, namely an explicit promise to only appoint justices who'll vote to overturn Roe v. Wade. Rudy could point out that pro-lifers have never gotten a promise like that from their pro-life GOP nominees, and could note that Ronald Reagan (Kennedy, O'Connor), George H.W. Bush (Souter), and George W. Bush (Harriet Miers) have all tried to put unreliable votes on the bench.
That, it seems to me, would have to be good enough for Dobson or anyone else. Rudy would be blatantly kowtowing, all the other candidates would need to follow suit, and the pro-life movement would have demonstrated its clout. That would create problems for Giuliani in a general election, but nothing like the nightmare of a third party challenge. So why doesn't Rudy do it? My guess is he's ornery and doesn't like the idea of kowtowing. But from the perspective of a Dobson, that's precisely the problem.