Everyone uses the slippery slope argument when it suits them and opposes it when it doesn’t.
It was used by the Left against the partial-birth abortion ban in 2003. It was used by the Right to oppose gay marriage in 2015. Both sides continue to use it today on any number of issues, from guns to immigration to the environment.
The slippery slope argument presupposes, usually without argument, a starting condition so precarious that the slightest tremor triggers a feed-forward avalanche, inevitably depositing us into the complete contrary condition from which we will be forever immovable, buried as we are under all that rubble.
In that way, the slippery slope argument is fundamentally superstitious, not to mention arbitrary and authoritarian, since by its very nature it denies problems rather than seeks solutions to them and opposes curiosity or experimentation of any kind. It is the intellectual and political equivalent of putting “Here Be Dragons” on a map. Whether there are any dragons is not open to question. You’re just not supposed to go there.
The argument, however, is not invalid — in the formal sense. A metaphorical slippery slope could exist. (I can recommend a good paper for anyone interested.) But that’s only true of its weak form. In its strong form, which is how it’s usually employed, it is no different than a false dilemma.
False dilemmas positively pervade politics, advertising, and polite speech. The reason is simple. They’re convenient. They pacify a complex and uncertain world. They take the crust off the sandwich of thought and peel its fruits and vegetables of their difficult-to-chew nutrients.
I suspect they’re also hard-wired into the brain, which we now know is a cognitive miser; it doesn’t want to think about anything it can invent a good reason not to.
Toward that aim, the slippery slope argument works better than a false dilemma for being deceptively cloaked. The slippery-slope arguer acknowledges, passingly, the existence of numerous possible alternative outcomes. Pragmatically, however, there are really only two: for or against, a tenuous present or a dystopian hell, with nothing between. A classic false dilemma.
In my experience, that is almost never the case. In my experience, the world is — except under certain rare fringe conditions — actually refractory to large-scale change, an uphill slope rather than a downhill one, where it gets harder and harder the more you push.
This is especially true in politics, where popular movements tend to weaken as early successes are achieved and the perception of risk abates, which explains why goal-directed change is rare in history — which brings up the second, latent function of slippery slope arguments: they empower a ruling elite.
If a slippery slope argument is true, not only is complexity denied and understanding irrelevant, dialogue is as well. There are only two choices, one of which we know to be right, so engaging on the other is a waste of time.
In that way, belief in the slippery slope is socially stable because it insulates the believer against alternative information. Group coherence is conserved, and the status quo–be it on the Right or Left–is reinforced, which is reassuring to in-group members.
This is why even small political battles tend to be championed on both sides as existence-level threats, with extreme labels applied to the opposition and internal dissent shouted down.
As such, I’ve found use of slippery slope arguments to be a fair proxy for small thinking.