As I crank up the MP level gradually, I find myself having different uses for acid rain. At lower MPs, it was the main damage spell to clear everything. At mid MPs, acid rain became more of a proc and heal than a killing machine, I had to introduce some other skill(s) to finish up the stray mobs and elites. Now at higher MPs, acid rain just tickles the mobs. I think the procs and bleed stacks, but not the acid damage, so casting multiple times is not as DPS-efficient as other spells. Perhaps the optimum use for acid rain is to cast every 3-4 secs, or whenever a boss does its animation for some big hit.
However, since acid rain is THE best proc, I am rather reluctant to remove it from my build altogether. An elite pack or whole group of mob on lock-down is very helpful to survival. I used to run with stun/blind/freeze, until a crafted glove proved too irresistible with 15k DPS gain. I do miss the lock-down, and may need to fall back on the stun gloves in the future. But for now, the higher DPS is helping me zoom around in mid-MPs.
Oh, if the Uber party does not have a CM Wiz, a proc doctor is the best alternative. Don't forget to hex that irritating ZK too!
If you play solo most of the time, equip your Scoundrel with a Buriza (chance to freeze), Hellfire ring (+IAS if possible), another ring with +IAS, and an amulet with +IAS and % chance to blind. The mobs will beg for mercy. haha.
Going the proc path definitely makes the WD class very fun.
Jimbob is our resident doctors' advisor and skywalker is our official scribe, so you best consider their advise :D