Why ban ToB?

[Unavenger]

If the GM doesn't give a valid reason--refuses to give any reason beyond "becuase I said so" or "I don't like that" then you can't be certain that they're going to respect you in-game.

Yeah, because everyone has to justify every preference they have all the time.

It gets tiring and sometimes you don't want to give your players an essay on why you don't like something each time it comes up.

"It's overpowe..." "YEAH WELL LOOK AT ME IF I DELIBERATELY BREAK A WIZARD THEN ITS STRONKER HOW BOUT THAT HUH!"
"Yeah right but I'm not talking about power cieli..." "THEN YOU MUST BE TALKING ABOUT POWER FLOORS WHICH IS A STUPID WAY TO MEASURE CLASS POWER!"
"No, I'm not talking about the power floor, I'm talking about my experience with real players..." "BUT ACCORDING TO THE FORUM IT'S COMMON KNOWLEDGE THAT ALL WIZARDS ARE SUMMONING ARMIES OF [INSERT CREATURE THAT I GUARANTEE YOU MOST PEOPLE ON THE FORUM ACKNOWLEDGE AS BEING REALLY CHEESY HERE]"

No, it's a lot easier to say a blanket "No" without having to justify it every time. The only reason I'm allowing PoW at all is because dealing with balancing a borderline-immortal level 3 character against relatively ordinary characters is actually easier than getting into these sorts of arguments with every other player who wants to use the system.