I've heard it also that after a few hours photosynthesis will go down in rate. I assume this is due to internal sugar storage whatevers are being filled up. There was that guy who did give light 4/4 4/4 4/4 each day and still successfully veg. But without a control-group-experiment we'll never know if that did also result in the same level of growth than the same plants would do if under 24/7 lights.
The book mentioned also stated that under 24/7 growth would occur the most with 18/6 suffering about 30%, and 16/8 up to 45%. If you do the math you'll see that it'snot even a linear loss - it's more than what would've been expected, the author claims that this is due to metabolic processes that first have to rise slowly once the light goes on again.
If a 18/12 or 24/12 or 36/12 lightcycle doesn't increase harvest-weight or cut down bloomtimes then there's no point to it. But I doubt it. IF the plant receives power from being under light for 36h and stay in flower it will try to use that energy, mostly in trying to form buds. I have a digital equipment and 2 similiar tents to try this out, but IÄm not going to flower anything for the next 6 months.
edit:
The book is also quite detailed about light-cycles and from what I can gather the author gives the impression that the more light a flowering plant will receive during its lightcycles the bigger the buds are. That is, some strains may only need 11h of uninterrupted darkness to bloom, and if that's so, the author recommends giving 13h light.
But how long could the light-cycle potentially and maximally be until the plant "thinks" it has to revert to vegging again? Could I, uhh, give 12h darkness only 1 time each week? The plants circadian rhythm only works in 24h steps so actually the whole lightcycle as illustrated above seems unnatural to me.