On a general note, I think the debate gets unnecessarily messy at times. People just can't agree on what the criteria for banning something are : brokenness ? Centralization ? Hax-inducing nature ? Generally harmful nature for the metagame, whatever that means ? And so on.
Re
Centralisation is a symtom of being broken, generally. Look at the effect that banning Salamance and Latias had in Gen 4 [Many pokemon dropped
drastically in use, even SCIZOR dropped about 10%] Possibly the only exception is Wobbuffett... who nothing can really be done about, so people don't bother.
Even look now. After Blaziken was banned, are you seeing less Azumarill and Slowbro? How about more Ferrothorn and Scizor?
Hax-Induceing Nature and being harmful for the metagame is a reason to be banned, however. Like the evasion items were, and Moody, which both put the whole game to a dice roll or two. Generally, these two overlap. After all, something which is broken is harmful for the metagame.
So, basically, things are banned because:
A: They are broken, thus, harmful to the metagame [Darkrai, Skymin, Blaziken]
B: They are harmful for the metagame, in a way that severely removes the effect of skill. [Moody]