DPS expertise is there so that people with low expertise get crushed by players with full expertise but people with full expertise don't do outrageous numbers in pve.This requiring properly geared toons to succeed in pvp.
As an above poster mentioned, full expertise verus full expertise the +damage and + damage resist cancel exactly.
Healer expertise is so that BW have a tuning tool to make healing less or more effective in pvp without it adversely effecting pve.
(Trauma - Healing_Expertise) is a co-efficient they can set to whatever they want to tune healing in pvp and only pvp.
The stat is there for BW, not the players.