I don't think there's any backstory beyond whoever wrote it not expecting anyone to want zebra on all the time.
In trunk r6100-6101, I changed the draw zebra option from a checkbox to an enum with options: Never, Shoot, Rec, Always.
"Shoot" is equivalent to the previous zebra ON setting. "Rec" displays it when the camera is in rec mode, and "Always" also enables it in playback.
This is similar to the options for histogram, although the exact behavior may not be the same, and the order is different to keep the first two settings compatible with existing cfgs.
However,
the digic 6/7 drawing code does not actually allow drawing/updating zebra and histogram at the same time, so this probably not useful for most of your cameras. Maybe I'll fix it, or maybe I'll give up and revert the whole thing
(edit: I haven't bothered merging to the ximr branch because of this)
Notes:
* Zebra does not draw in alt mode, unlike histogram when set to "rec" or "always"
* The existing logic used for the "Shoot" option is only applied to still (not video) shooting modes. "Rec" and "Always" are applied regardless of mode.
* Zebra in playback mode behaves like activating with half press did, meaning that if the image isn't full screen, stuff outside the image will be treated as underexposed.
