Excellent question, in my opinion courage of the colossus isn't the end all be all ability, bond of stone is an excellent keystone mastery for a support. Tham Kench can use bond of stone safely because of his health, so if you do not feel like courage of the colossus is working then the next thing I would recommend is bond of stone. However the reason I pick colossus is because of the extra cc and the shield you get can save your life, and the stun can also save your life, your ADC's life or can secure a kill.