It's not effective at all, that is the point. There is no other template or skill that allows a person to use a 2-3 second spell to remove everything and heal. No matter what circle it is. Aside from this, people run, they're not going to sit there and let you disrupt them from healing. The difference being a regular mage would take several spells to remove poison and heal. A dexer of any form will require a combination of aids, apples, or pots to do the same.
Also throw in sleep, which slows you down and removes the active combat vs person you're fighting in a couple seconds, and 45 dci there's not going to be any problem casting cleansing winds. It isn't that people aren't trying to interrupt the spell or are doing something wrong. When a broad spectrum of templates are facing the same problem again 1 template, it's not likely it's everyone elses fault.
Sleep lasts between 1-3 seconds unless you don't have decent resist spells. And has an immunity duration after. (Don't know what sleep has to do with the CW issue other than it allows the mystic to get one off guaranteed)
Running is EVERYBODY's defense. And any caster has to stop to cast, so if they are running they are neither defending nor attacking.
Bandages -> Healing on the Run -> Trade off, Use Other Consumables to remove curses, 4-8 seconds time
Cleansing Winds -> Healing Everything ->~2.5 sec Longest Casted Heal time in Game by 1 second, Interruptible
GHeal -> Heals for 6-10 Less than an reduced CW , 1 second less cast time, Interruptible
Lesser Heal -> Heals for 9-18 .5 second cast time, Interruptible
Spirit Speak -> Heals for 20-39 Same cast time as GHeal, Heals Through Poison, Mortal, Interruptible
Close Wounds -> Heals for up to 40, Cast time between 1-1.5 seconds, Interruptible
Heal Stone -> Instant Healing, Similar to potion, Up to 300 HP total healing. 15 Sec Recharge to max effectiveness. (Invokes Heal Pots Cooldown)
If you don't hit a mage in 2.5 seconds, his 2 heal combo will heal for more than Cleansing Winds
If you don't hit a Paladin in 2.5 seconds, his 2 heal combo will heal for more than Cleansing Winds
If you poison a mage and dont hit him for 2.5 seconds, his cure/heal combo will heal for more than cleansing winds
If you poison a paladin and don't hit him for 2.5 secodns, his cure/heal will heal for more than cleansing winds.
If you mortal a mage, he will be unable to heal for 6 seconds unless using a consumable.
If you mortal a Paladin and don't hit him for 2.5 seconds, he will heal for more than cleansing winds
If you mortal a mystic and don't hit him for 2.5 seconds, he will be able to CW and then heal himself
2+ Mystics, healing 1 person, IS extremely powerful. Simply because CW effectiveness is realized with multiple castings. I think this could be addressed by one ore more tweaks, to mana cost, lower effectiveness after debuff removal, and maybe even that diminished returns mentioned, though probably less so that one. Due to the fact that in 1v1 CW has more limitations, much (not all) of the difficulty can be dealt with by adaptive playstyle.