This is about money. The more possible combinations you have going, the greater your chances of winning, albeit it winning less than if you played one bracket in many different pools.
Look at it this way: If you're only gonna play one combo, find one pool where the entry fee is larger and the prizes are bigger, and go with that one.
But if you're in more than one, it's silly to play the same entry.
If this were a game where there were only, I dunno, 20, 30, or 40 possible outcomes, and you felt strongly about one, and the payoff was greater than 20, 30, or 40 to 1, that would be one thing.
But there are how many different possible ways are there to fill out a 64 team bracket. Jeeze... 2 to the 64th power (I think) in the first round alone. That's like more than a trillion combinations right there. Even if you throw out the combos where a 16 seed beats a 1, still...
Even if you say there will be no upsets until the 7 vs. the 10, and the 8 vs. the 9, there's eight of those matchups, so the chances of getting them all right are 255-1. And we're not even out of the first round. wait until the later rounds, when it gets harder.
This game is basically all luck. Play the percentages, man.