While it's definitely healthy to step back and attempt a broader view of any issue, it seems to me that "patent troll," by admittedly informal definition, means a non-practicing entity litigating solely for profit.
Maybe I'm missing something, and maybe it is a lot more complex than that, but I think this is one case where ill-defined or over-applied terms may be muddying the water with regard to what is or isn't taken into consideration on an issue that, by its nature, seems to have no redeeming value.
Read the papers to get an idea of how they are beneficial. One obvious way is straight-up helping individual inventors or small companies take on big ones. But another less obvious way they help is by creating a secondary market for patents. This has many follow on effects. For one, VCs are more likely to fund startups with the idea that they can partially offset the risk of failure by monetizing the patents (papers by Merges, Mann, etc. to this effect, IIRC). This enables entrepreneurs to pursue riskier technology ventures.
Saying they exist only to litigate is missing the whole picture. That is simply what they specialize in, primarily because patents are essentially a right to sue, and there's a gap in the market when it comes to enforcing patents.
Now you may say that patents themselves are crap and represent no value, but 1) that's a different problem than trolls, as others have mentioned, and 2) you'd be making a generalized assertion with little data to back it up. Even if you are an experienced patent litigator who's seen a lot of cases, there are biases at play. Which is why, again, we should turn to studies, which, again, do exist and, again, present a much more nuanced reality.
Maybe I'm missing something, and maybe it is a lot more complex than that, but I think this is one case where ill-defined or over-applied terms may be muddying the water with regard to what is or isn't taken into consideration on an issue that, by its nature, seems to have no redeeming value.