Agility can, and should, be wrapped into the dexterity stat. Being agile and dextrous kind of run hand-in-hand. Here's the the thing, taking evasion out via stats and leaving it as an independent stat found exclusively on tank gear means you can leave it as it is stat-wise. Otherwise you run into the scaling "rating problem" that a lot of MMOs have found. As you level, the value of the rating decreases. If you're stacking agility to become evasive, you end up becoming weaker down the road. Also if, a certain race has the highest natural agility, it will default to the most evasive tank. As damage reduction / health can only reduce so much, but evading an attack causes 100% damage reduction of said attack.
One argument could say you're making all the classes the same... And in a way you are, but also you're helping to avoid item design issues. If you have 10 core stats that are important, you end up having to design 10 well-designed pieces of armor for every slot, or you design 4-6 pieces per slot (PPS) and have cross stats that aren't useful on each piece of gear. So instead of designing a dex set and an agi set, you design a dex or agi set, and you cut out the number of sets in the game which simplifies loot tables and reduces the chance of piece of gear dropping that no one can use.
WoW Blackhand-US-Date of Retirement: 9/21/2010... /Sigh
Devari - 90 Rogue 85 DK Druid/Mage/Warrior 70+
FFXI - Shiva "Retired.... Or not? One more try, honest."
Desmar - 65 Sam 36 Mnk 18 Thf 12 War
FFXIV - Devari Garamond - Sargatanas 50 Paladin / Culinarian / Weaver / Armorer
Beta - Devaria Ariadne - Ultros - Pugilist