As a Disc Priest, healing in 25M, I find the cloak to be fantastic. I do not understand what you're complaining about, free extra healing?
The least % of the healing done on it was on Norushen, 4.6% where I practically ONLY smited the boss. On the other fights it was about 5% of my healing done, to an amazing 6.6% on Protectors. Same with the other two disc priests in my guild, it seems to be way more than the 1-3% that people are talking about. Maybe you're not putting it to use good enough, or maybe you're just being VERY VERY unlucky with the procs.