Having level 40's fight level 50's is stupid because you would require 22% hit. Have you ever tried to perform any semblance of a rotation on a +10 level dummy and seen 1 out of every 5 special attack miss? It would make for completely idiotic and frustrating gameplay.
What kind of challenge is that? "Hope a DD attack misses instead of the attack that refreshes the debuff" is not compelling or challenging gameplay.
The downside is putting a horrible broken piece of frustrating gameplay into the game and calling it content.
edit: BTW, the 22% hit needed is only for physical attacks. As you can see from the chart on this page:
http://www.wowpedia.org/Spell_hit spell hit scales much more steeply. Against just a +5 mob, you have a 39% chance to miss. I imagine against a +10 enemy, your mage might as well just run in and melee because the spell miss chance is so high it's not worth even trying.