I have moved down in limit once and i don't wanna do it again, it costed me 3 months X(. I had 300BB rule for FR and going from 0.1/0.2 to 0.25/0.5 with 150$ and dropped to 60$, and allmost quit poker, then i decided to set higher bankroll limits and took me 3 months (i didnt play much) to climb to 300$ and then move up to 0.25/0.5 again.
Another reason for quite big bankroll is that you can overlast your bad run and stay on that limit, and if u don't you can be sure you don't play well enaugh and move back down and still have bankroll to play comfortable at the limit below eg. you move up with 1000BB and loose 600BB, you can be pretty sure there is something wrong with your game and move bach down a limit at which you have comfortable 800BB.
Secon example: you move up with 500BB, hit 300BB down, have to move back, loose another month or two to grind it back, not knowing if it was your play or just bad luck.