It seems to me that the skillpoint investment is huge for such a small debuff.
It’s only -25% with 12 points in the last modifier plus at least 1 for the skill and more if you want the increased radius/duration (which you most likely need a bit of given how small the starting numbers are).
What do you think?
Well, two things. One, RR is always worth it.
Two, it’s RR for damage types that are hard to stack RR for, namely Aether/Chaos. Aether less so, as Chaos in particular is one of the worst damage types partly because it lacks significant reliable RR in sizable doses (among other problems).
Wut?
What about mines? The only problem I find with Chaos is the %racial damage isn’t found in any occultist/inquisitor relic
It definitely is. While playing my DW, I noticed significant increase in killing time, when I was walking through Malmouth in Elite.
It can also be spammed, and is an AoE spell. It’s fantastic.
I’m trying my hardest to theory craft a build centered on Word of Pain, just because I love it 
I just need to work around on the scaling problem end-game.
Yes, Word of Pain isnt the most useful RR skill. Points into base skill increase duration only, points into one mod increase radius only, and the last mod adds RR only.
If you take CoF, investing into base skill adds AoE, duration and RR for physical (and slows enemies!).
However, RR is still worth it. Yes, you probably wont use Word of Pain on all mobs (while CoF can be used on everything - you can debuff a huge pack with 1 click). But you will still use it on bosses, and it will be still worth it.