I think the rare earth equation is missing a factor. Any intelligent species will at some point discover nuclear weapons. So the missing factor in the equation is the percentage of species that survive the discovery of nuclear weapons versus those that destroy themselves.
Are you referring to the Drake Equation?
Actually, this is rolled up into the consideration of lifetime of a civilization, or "L". I took a very interesting seminar on the topic of extraterrestrial life in college. I imagined I would watch a few episodes of Ancient Aliens and write a few papers about it, but we broke down the Drake Equation to the nitty gritty.
The lifetime of a civilization can be limited by any number of things which fall into only two categories. Either we'll kill ourselves off or something else will do it for us.
It's easy to come up with ideas for how we'll end our species, and nuclear weapons is just one of many possibilities. It also includes biological weapons, climate change, the singularity, etc. Granted, some are more far-fetched than others, but that's what's fun about pondering the possibility.
Coming up with ways in which other things could kill us is tougher but scarier. Seismic activity, massive solar storms, asteroid impacts, devastating plague, are the ones that might come to mind, but there are a whole host of astronomic phenomena we don't understand. You can even add in alien invasions. It's theoretically possible, right?
But the flip side to the idea that every advanced civilization eventually builds the tools to destroy itself is that to have advanced so far, we must also be building the tools to save ourselves. And we are. Nuclear weapons have devastating consequences, but nuclear energy offers a lot of solutions to climate change, if done correctly. Biological weapons and incredible medical advancements also go hand in hand.