And that's exactly my point.
In short, Soul Harvest amps damage (and to a lesser extent, magic resistances) to the point that it's difficult to justify any Witch Doctor build without it. It appears that the returns do diminish, but it's still a massive damage amplification.
While I like the skill, I'm concerned about the existence of "must have" skills in this skill system-- I'd much rather see a wide variety of viable builds that incorporate the whole spectrum of skills, rather than a few builds that all incorporate Soul Harvest.
Unfortunately, I'm not sure how to fix it. I don't want to see it massively nerfed, but at the same time, it's so good at the moment I can't justify playing without it.