That is exactly our intention. We want every one to think about and reflect his or her own values. We believe there can't be the single one truth for all. But the real danger is, where people don't think at all about what's right or wrong.
Symbolics like this pledge might help with the former, because it raises the idea that software may have an ethical component.
For the latter: its kind of sad that these people at yahoo – being incredibly more employable than average – still couldn't muster the courage to say no. Could've done it quietly and found a job somewhere else. Could've done it publicly and be a hero (and probably still find a job – some companies out there would've wanted to support them, or use their hiring as marketing).
For this tactic to achieve its goal (that the software doesn't get written), you'd have to ensure that literally zero developers now and in the future would be willing to write the code. As was discussed in a previous thread [1], when faced with the prospect of writing "unethical code" the consensus seems to be even if a few developers take an ethical stand, there will always be some other developer out there without such strong convictions willing to write the code instead.
I'd also like to point out that being employable does not necessarily mean you're in the position where you can put your job on the line for something. I consider myself an "employable" software guy but I've got bills to pay, and 3 months of unemployment while I find my next job is not exactly compatible with that.
There are certainly some fundamentally-broken things that will happen as long as anyone is willing to do them - I don't expect spammers and ad fraudsters to disappear because a few programmers quit in a burst of ethics. But Yahoo wasn't dependent on unethical behavior, they had a (somewhat) viable business doing reasonable work.
Those are the companies susceptible to quiet moral stands. They're not dependent on ethical lapses to survive, so they can push back on them and continue to exist. And they're big, wealthy companies that spend lots of money on getting good programmers, so if some of those talented programmers say "screw this" and quit, it's a problem even if they can replace the missing bodies.
None of this addresses the last point - I don't expect people to starve over this, and I imagine most would stick around until they had a new job lined up. But I do think that pushing back on these companies can help even if there are people willing to do the dirty work.
Everything is more complicated than this.