I mostly agree, but I think the slogan (like, I think, many others about which similar things could be said) has some value none the less.

A logically correct but uninspiring version would go like this:

It is a common human failing to pay too much attention to safety and not enough to liberty. As a result, we (individually and corporately) will often be tempted to give up liberty in the name of safety, and in many such cases this will be a really bad tradeoff. So don't do that.

-- Not Benjamin Franklin

Franklin's slogan serves as a sort of reminder that (1) there is a frequent temptation to "give up essential Liberty, to purchase a little temporary Safety" and (2) this is likely a bad idea. Indeed, the actual work of figuring out when the slogan is appropriate still needs to be done, but the reminder can still be useful. And (3) because it's a Famous Saying of a Famous Historical Figure, one can fairly safely draw attention to it and maybe even be taken seriously, even in times when the powers that be are trying to portray any refusal to be terrorized as unpatriotic.

Of course Volokh is aware of the "reminder" function (as he says: "The slogan might work as a reminder") but I think he undervalues it. (He says the "real difficulty" is deciding which tradeoffs to make, but actually just noticing that there's an important tradeoff being proposed is often a real difficulty.) And, alas, its Famous Saying nature is pretty important too.