As such, because all of those variables are next to impossible to quantify and balance around while still preserving PvE performance, it's simply a truth that you have to accept as a hard core PvPer: some classes and specs are just not viable.
No. That is a lazy man's excuse for bad PvP balance. There are plenty of methods to enhance classes that are bad at PvP but already competitive at PvE. The most egregious (one full YEAR) case of BW leaving a class worthless in PvP is Merc dps. And yes, Merc dps is fine for PvE. But BW has gone out of its way to nerf PvP traits for Merc dps (i.e. Sweltering Heat). You can not explain this by simply waving your hands in the air and exclaiming that there are too many variables to account for. BW could easily buff underpowered PvP dps classes by adding a +10% resistance to damage when stunned or cc'ed to the top tier talent for these classes. This would have virtually no effect on endgame PvE.
The problem is far deeper than an inability to accounting for multiple variables. The problem is that BW has improperly diagnosed which subclasses need buffs and which subclasses need nerfs in PvP. The ludicrous buffing of Rage/Focus dps in 1.6 is just one example. Until BW catches up with the knowledge base that exists amongst the top PvP players, PvP balance will always be lacking. And the refrain of "there are too many variables, so I can't fix it" will always be weakly offered as an excuse.