Originally Posted by
jbmasta
My theory is that the amnesia was deliberately included as a safety feature, to prevent earlier incarnation deliberately bring about the series of events leading to the multi-incarnation interaction, avoiding further complicating an already timey-wimey situation.
It also ties into how what is observed of one's personal future then becomes set in stone, and must happen regardless of whether its good or bad (Angels Take Manhattan). If the War Doctor remembered Gallifrey being saved, then there's no guilt in Nine, Ten and Eleven (which is the reason they forgot about him), which he saw them display, therefore remembering the actual fate of Gallifrey would screw with his future timeline. Additionally there's that old chestnut of the predestination or self-fulfilling paradox, bringing about an occurrence you're trying to prevent. Using knowledge of the future to negate it. And seeing how your future self turns out is a big dollop of future knowledge.