They were designed this way to make it easier for the FD to extract passengers, especially in places like CA that prohibit the key in the door, or in situations where FD has the wrong key. Think of it as a more evolved version of a retiring cam setup (i.e old style Westinghouse).
I just had one change of mind about the harmonics. One advantage they have is that they do much much better on antique heavier door panels than belts. The chain drive gives better torque and reduces slip. Another is that they can be quieter on extra large openings than linear. They also don't have to relearn after power failure due to the use of cam controls. Most of these pluses only apply to very old retained equipment.
What's more interesting is that GAL just release their LWL clutch for harmonics. This greatly eliminates a lot of the aforementioned issues and replaces the gate switch w/interlock. Great step in the right direction, since there are so so many GALs in the NA market. The only two things I don't like is that it reminds of of the now obsolete Schindler QKS9 (common in AU & NZ on their M-series), which had lots of problems and that the doors are still splitable.
https://www.gal.com/products/door-equipment/lwl-clutch-and-interlock/
