So most people build with at least Strong Draw, which means Pen > RoA is typically cheaper than IMS in terms of focus (5 to 6 -- 4 to 6 if you trait Arrow Storm too) but even with everything untraited, the focus cost is equal. Granted, actual power cost favors IMS, but unless you need the corruption removal, IMS is only beneficial if you crit.
I went thru a bunch of the parses I've put together over the past couple months and tabulated my numbers for Pen, RoA & IMS, here's what I found:
Pen>RoA wins by 40.26 points (542 Pens, 152 RoAs, 85 IMSs)
RoA wins by only 3.6 points (710 Pens, 265 RoAs, 51 IMSs)
And that's only counting 1 hit per RoA, so if you have multiple mobs (and AoE is allowed), Pen>RoA pulls ahead even more.
Also, IMS isn't affected by oils, so if you're getting more damage from fire or light oil, that adds to the gap (the numbers ahead are strictly w/o oil)
(note, I haven't looked at why IMS fared better in relation on beta.)
If RoA is on cooldown, or you can't use AOEs, IMS is still worthwhile. Obviously, if you need the corruption removal, IMS is our only option. But in many cases where we would use IMS, we're actually further ahead much of the time using Pen>RoA. One other exception would be if you had enough focus for Pen>IMS. That comes out ahead.. but then again, if that were the case, you could do Pen>ISB > Pen.
IMS is like HS's faster little brother. They both occasionally give some big, pretty numbers for people to ooh & ahh over, but neither really pulls its weight in the DPS department.