I find it unlikely that scientific secrecy is never the right answer, just as I find it unlikely that scientific secrecy is always the right answer.

Sure, you can find exceptional scenarios where secrecy is appropriate. For instance, if you were a scientist working on the Manhattan Project, you certainly wouldn't have wanted to let the Nazis know what you were doing, and with good reason.
But barring such kind of exceptional circumstances, scientific secrecy is generally inappropriate. You need some pretty strong arguments to justify it.

If the investment is low, it is less likely that anyone will come up with the same discovery and so you may want to keep it a secret.

How much likely it is that some potentially harmful breakthrough happens in a research field where there is little interest?

psychologists work with a lot of different theories of the mind, many of which do not explicitly endorse incremental theorizing

Is that actually true? And anyway, what is the probability that a new theory of mind is potentially harmful?

Honestly, I find cases of alternative pleading such as V_V's post here suspect. It is a great rhetorical tool, but reality isn't such that alternative pleading actually can map onto the state of the world. "X won't work, you shouldn't do X in cases where it does work, and even if you think you should do X, it won't turn out as well" is a good way to persuade a lot of different people, but it can't actually map onto anything.

That statement seems contrived, I suppose that by "can map onto the state of the world" you mean "is logically consistent".
Of course, I didn't make that logically inconsistent claim. My claim is that "X probably won't work, and if you think that X does work in your particular case, then unless you have some pretty strong arguments, you are most likely mistaken".