Plague and rabies

https://d2.lc/AB/wiki/index3a3a.html

Essentially what happens is that everytime your merc procs Poison Nova, that Nova will kill your Rabies damage for one frame. After that frame, your Rabies dmg resumes as it is bound to be higher than the Nova proc.

Realistically, a Frenzy merc with dual Plague will hit at 10 frames for 2 hits (you can push that to 9 but let's assume you won't, seeing how it'd be counterproductive).
So that's an average of 5 frames per hit. Poison Nova has a 25% chance to proc on each such hit so again on average, you'll get a Nova every 20 frames.
As a result, you would lose 1 frame of Rabies every 20 frames or simply put, you would lose 5% of your Rabies damage to the merc's Poison Nova procs.

The question is hence simply "does LR proc often enough and have enough of an effect to outweigh 5% dmg loss?".

Though the 5% dmg loss isn't really 5% but likely quite a bit lower because 5% would assume your merc hits 100% of the time which we know he won't. Block, evasion, shitty hit rate, all of those suddenly work in your favor as they reduce the total number of poison novas going around. Though they obviously still work against you yourself so not entirely in your favor.