Sometimes, almost always from dubious sources, I see a PS curve defined in a way that one of the Bézier Control Points is placed fairly normally, or maybe even a bit exaggerated… and the other one is hidden—it turns out to be a zero-length BCP.
In some (most?) cases, this is clearly a Bad Thing. The curve is such that it should be more balanced, and it isn’t.
But in _some_ cases, it isn’t such a clearly bad thing from an appearance point of view. Something that starts out closer to a straight line and gradually curves more seems ~ reasonable. Rather like a spiro curve. Although, in such cases, usually a short but not zero-length BCP would also work pretty well.
I hae to admit, in terms of BCP criss-cross concerns, I don’t even know how to evaluate a zero-length BCP! Is it impossible for it to go wrong? Is it always wrong? Heck if I know.
Are there other technical or rendering-efficiency reasons to avoid a zero-length BCP on a curve?
(Of course, if you have _two_ zero-length BCPs defining a curve, you get a straight line; no reason to have a curve at all, unless it is for compatibility in a variable font.)