> Do any programs actually read the italic angle info in the font and what do they use it for?
Yes. For example, I have seen OTF fonts incorrectly sorted in InDesign's font menu, and the culprit* was a faulty Italic Angle value -- the Italic Angle of the "Italic" font was set to 0 (zero). The value on the other italics in the family (e.g. Bold Italic, Semibold Italic) was also incorrectly set to zero, but for some reason the sorting of those fonts was not affected. Only the plain "Italic" was sorted out of order.
*The fonts' naming was all correct BTW, so that wasn't the problem