Certainly some slippery slope arguments are good, though!
For instance, suppose a terrorist starts demanding a large sum of money not to blow up a plane. Suppose also that the direct risk-corrected value of the plane, even excluding passenger lives, also exceeds the price the terrorist is asking for. We still make it policy not to negotiate with the terrorist, because doing so would create a positive feedback loop that pushes us down a slippery slope.
When you think about it, it's a fairly insidious argument to say that slippery slope arguments should be rejected because they fall into a category of arguments that are subtle and hard to refute because they are so insidious. An amusing implied prior of the slippery slope fallacy is that once we start accepting valid slippery slope arguments, what's to stop us from accepting the really terrible ones that take the same structure?
> it's a fairly insidious argument to say that slippery slope arguments should be rejected
Of course it is insidious (and no true Scottsman would say such thing!) This is because the actual fallacy is a false slippery slope. The essence of the error is failure to demonstrate that the actual slope exists. If you demonstrate the causal relationship between the first step and the rest of the slope then it is no longer a false slippery slope.
The internet seems to be a fan of being overreaching in proclaiming fallacies in my experience. Sometimes it feels like you can find at least one misuse of "ad hominem" on every comment thread on reasonably popular sites. Similarly, many people like to dismiss arguments by just saying "slippery slope" and moving on.
As you mentioned, slippery slopes are not inherently invalid. In fact, the real fallacious use of a slippery slope is hardly related to it being a slippery slope at all!
Assume a slope A->B->...->Z. Further, assume that each step of this chain is known to be true with 100% accuracy. That is to say that we can perfectly predict that if A happens then B will happen and if Q happens R will happen. Thus, simple transitivity shows that if A will happen Z will definitely happen. If Z is complete destruction of the Earth you probably won't find many people saying that it will be perfectly okay if we do A because slippery slopes are invalid.
However, slippery slope arguments often have a much smaller probability of occurring at each step of the chain. You often get things like "if we don't have a death penalty then people won't be afraid of killing people so murder rates will go up so you'll get pregnant and die". Clearly there is no part of this chain that occurs with probability 1 (except, as Mean Girls tells us, getting pregnant->death). In fact, most of the chain is fairly low probability and composing them as such makes it even smaller.
To make this a bit clearer again, let's take our A->B->...->Z chain and make each step of the chain have a .9 probability. If A happens, B will occur with .75 chance and if Q happens, R will occur with .75 chance. Since there are 25 hops on this we can see that the probability of Z occurring given A is actually .75^25 = .00075 (ish). Even if each individual step seems reasonable (I mean, hey, it will happen 3 out of 4 times!) the full composition is just laughably unlikely.
However, it seems that GP was actually talking about a continuum fallacy rather than a slippery slope as we've discussed.
Secretiveness, in this case, is a mechanism to avoid the slippery slope. As a sister comment to your comment points out, often other effects will step in so we don't go all the way down the slippery slope, and the "secret ransom" is one such effect.
Personally, I feel slippery slope is among the worst rebuttals to an argument and brought out way too frequently on HN. "Y is wrong, and X comes before Y, therefore X is wrong too."
For instance, suppose a terrorist starts demanding a large sum of money not to blow up a plane. Suppose also that the direct risk-corrected value of the plane, even excluding passenger lives, also exceeds the price the terrorist is asking for. We still make it policy not to negotiate with the terrorist, because doing so would create a positive feedback loop that pushes us down a slippery slope.
When you think about it, it's a fairly insidious argument to say that slippery slope arguments should be rejected because they fall into a category of arguments that are subtle and hard to refute because they are so insidious. An amusing implied prior of the slippery slope fallacy is that once we start accepting valid slippery slope arguments, what's to stop us from accepting the really terrible ones that take the same structure?