The hold-up with this specific aspect of the software is not a conceptual one. I don’t think anyone would be able to produce a strong argument for saying the Format Bar should not be customisable, it’s just that it can’t easily be, given its underlying technology used to create it, and providing a format bar that is customisable would require more work than would really be worth the effort (as nice as it would be, so that should put into perspective how much work we’re talking about).
In fact, the Windows version does have a fully customisable format bar, with buttons for nearly all of the main formatting commands. So again, it’s not that there is a conceptual opposition to the problem, it’s that on the Mac, this is a somewhat fragile manipulation of the built-in formatting controls provided by the text engine.
This is also, incidentally, why there are bizarrely out of place formatting commands in the main toolbar. 
Update: I stand corrected, it is apparently common for some formatting commands to be in the main application toolbar, but not all. I do not attest to being a toolbar expert, myself.