The question is how much, and what to do about it. Day and Schuster argue in their paper that the issue is patent thickets, as their abstract shows. The draft article Patent Inequality, is on SSRN:
Using an original dataset of over 1,000,000 patents and empirical methods, we find that the patent system perpetuates inequalities between powerful and upstart firms. When faced with growing numbers of patents in a field, upstart inventors reduce research and development expenditures, while those already holding many patents increase their innovation efforts. This phenomenon affords entrenched firms disproportionate opportunities to innovate as well as utilize the resulting patents to create barriers to entry (e.g., licensing costs or potential litigation).
A hallmark of this type of behavior is securing large patent holdings to create competitive advantages associated with the size of the portfolio, regardless of the value of the underlying patents. Indeed, this strategy relies on quantity, not quality. Using a variety of models, we first find evidence that this strategy is commonplace in innovative markets. Our analysis then determines that innovation suffers when firms amass many low-value patents to exclude upstart inventors. From these results, we not only provide answers to a contentious debate about the effects of strategic patenting, but also suggest remedial policies to foster competition and innovation.The article uses portfolio sizes and maintenance renewals to find correlations with investment. They find, unsurprisingly, that the more patents there are in portfolios in an industry, the lower the R&D investment. However, the causal takeaways from this seem to me to be ambiguous. It could be the patent thickets that cause that limitation, or it could simply be that industries dominated by large players are less competitive and drive out startups. There are plenty of (non-patent) theorists that would predict such outcomes.
They also find that firms with large portfolios are more likely to renew their patents, holding other indicia of patent quality (and firm assets) equal. Even if we assume that their indicia of patent quality are complete (they use forward cites, number of inventors, and number of claims), the effect they find is really, really small. For the one reported industry - biology, the effect is something like a -0.00000982 percent likelihood of lapse for each additional patent. This is statistically significant, I assume, because of the very large sample size and a relatively small variation. But it seems barely economically significant. If you multiply it out, it means that each patent is 1% more likely to lapse for every 1,000 patents in the portfolio (that is, from 50% chance of lapse, to 49% chance of lapse. For IBM - the largest patentee of the time with about 25,000 patents during the relative time period, it's still only a 25% change. Most patentees, even with portfolios, would be nowhere near that. I'm just not sure what we can read into those numbers - certainly not the broad policy prescriptions suggested in the paper, in my view.
That said, this paper provides a lot of useful information about what drives portfolio patenting, as well as a comprehensive look at what drives maintenance rates. I would have liked to see litigation data mixed in, as that will certainly affect renewals one way or the other, but even as is, this paper is an interesting read.