To be frank, I notice zero difference between the requirement to attack the rear versus side.

Mechanics are going to happen either way. Mechanically spamming DK is going to flub the flank requirement and drop the potency to 100 more or less as often as failing the Bootshine (which has a lower penalties when you fail it compared to DK, ironically).

Actually, that pretty much closes this discussion point: interleaving Bootshine is a DPS increase not only in general when you succeed, but also when positioning is difficult due to mechanics and bosses that like to turn and cast spells, since DK's potency penalty when you mis-position is pretty major.