That's a good point, for example when many individual app developers were being sued / threatened for the use of in-app purchases, it could be argued that the root of the problem is that something like in-app purchases is not an idea that otherwise wouldn't see investment due to the lack of a profit motive, and that it shouldn't be covered by the patent system in the first place.

For the idea that patent trolls add liquidity to the invention marketplace, I'm under the impression that the cost of acquiring the patent in question is often only a minor component of the operation, and the major cost is litigating it. Which seems to suggest that the primary economic effect is something else (what that is I'm not sure).

I think part of the problem with NPEs is that in practice there is a natural disincentive for large companies to sue over (possibly) frivolous patents, since they generally each have large patent arsenals and can countersue causing a loss to both companies. So there is an incentive to have some degree of restraint except in particularly egregious cases, and for total patent wars not to go on forever. NPEs obviously don't have this incentive, and it's not clear to me how a change in patent law could effectively replicate it.