So, here's the deal. If you completely separate a population, it may drift apart. Platinum in NA doesn't mean the same as Platinum in OCE, Master in LAN doesn't mean the same as Master in KR. Even within a server, if half of the population only plays during the day and the other half only plays at night, they may also drift apart. But since obviously not everyone picks the same time of day or days of the week to play (and some people play all day and all night long), there's a lot of bleeding over between times.
So the effect of playing at a different time of day or day of the week is minimal. If you were going to try to play during the best time anyway, you'd want to play with the population that is the WORST at the game, not the best. Think about it: if you consider Diamond in KR to be far better players than Diamond on LAN, would you rather do your promos to Diamond on the KR server or the LAN server? You'd get crushed playing with the KR Diamonds. You want everyone to be as bad as possible so that your team has an advantage—because they have you.