If you're using your follower for damage, you are doing it wrong. Followers cannot ever do enough damage to be helpful beyond low levels, so you give them crowd controlling abilities to help group/slow/debuff, etc.
Using broken promises on a follow makes sense if the follower has other items that trigger "on crit". For example there is a weapon that has a chance of dropping a health globe on crit. When combined with broken promises, it produces a fair number of health globes.