Oh, I agree that people don't really have a right to complain about CC's effectiveness if they willingly do not use the countermeasures available to them. That wasn't the point.
The point was that not everyone builds Tenacity, period. Some champions can't afford to fit it in their build (Vel'koz can't really afford to lose MPen for Tenacity, most Marksmen can't afford to swap Berserker's or build a Zephyr in place of other items). Some players play based on avoiding CC rather than weakening it; that doesn't mean they never get CC'd.
Telling me that I "have to" account for Tenacity when I have a roster of 120+ champions with multiple playstyles apiece is like telling me that I can't compare damage output without accounting for defense items and resistances. What guarantee do I have that my opponent will or will not build Mercury Treads or Zephyr (or if you're on TT, Moonflair Spellblade)?
I'm not saying that balance is done without regarding these effects, but I am saying that you can't guarantee that 100% of scenarios will involve them. It's easier to calculate based on the default scenario--that they have no Tenacity--if it's not something innate to every champion.