I think it's useful to differentiate between "arguments" and "debates". An "argument" is a meeting of the minds, grounded in mutual respect, in which each side has an opposing a priori belief that they might be willing to change. A "debate" is a zero-sum game in which each person arguing is trying to change the mind of some third party^1. People often call interactions "arguments" when, in fact, they are actually "debates".

Doubele-cruxing can be useful in arguments, but the audience of a debate should approach any crux with incredulity. Anyone with half a mind toward "winning" a double-cruxed debate would choose an untenable position as their "crux". A practiced debater will find the scientific concensus and then work backwards to a crux. Not the other way around. Then they get the rhetorical benefits of acting "reasonable" without actually giving up anything useful to their opponent. In fact, if their opponent is clueless enough to accept the crux, they've just won the game. And if their opponent complains, well now the opponent is "meta-debating" which is apparently bad thing (and is a bad thing anyways because it makes you sound like an egghead).

So, double-cruxing is a great strategy for an argument between people who mutually respect one another. But audiences to a debate should be careful about whether they're being hood-winked by a rhetorical trick.

In short, a lot of this philosophizing about how to have a good argument really just boils down to "have the right argument with the right person". Two people who respect each other talking about something that both have fluid opinions on rarely devolves into a useless conversation.

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^1: A debate in which there's no third party present might be called "a waste of time".