Patent pools have many flaws, which may explain why they have been used so infrequently. [http://www.theglobalipcenter.com/sites/default/files/reports/documents/Prizes__Patent_Pools.pdf (GIPC)]
'''Elimination of Competition''' Opponents criticize patent pools for the potential of anti-competitive behavior and collusion, primarily with regards to substitute patents. Substitute patents cover competing technologies that can be developed in parallel without risk of infringement [http://www.wipo.int/export/sites/www/ip-competition/en/studies/patent_pools_report.pdf]. Patent pools formed between entities holding substitute patents eliminate competition in that particular technological sector, reinforcing the parallels drawn between patent pools and cartels [http://www.wipo.int/export/sites/www/ip-competition/en/studies/patent_pools_report.pdf (WIPO)]. Patent pools potentially create a way for companies to share competitively sensitive information, such as pricing, marketing strategies, or R&D information among its members." [http://www.wipo.int/export/sites/www/ip-competition/en/studies/patent_pools_report.pdf (WIPO)] For example, R&D information and developments could be shared in grant-back provisions, a requirement that companies share new or downstream technologies and innovations with members of the pool without additional fees.
'''Licensing Practices''' If a patent pool restricts its members from licensing its patents independently, it lowers the incentive to produce alternatives and inflates the costs of goods or technology for consumers. The Department of Justice and Federal Trade Commission state that restrictions on licensing may create "a barrier to entry if existing relationships make it harder for 'new firms to come in and overcome the patent thicket'." [http://www.justice.gov/sites/default/files/atr/legacy/2007/07/11/222655.pdf (DOJ)]
'''Pricing''' Given agreements between patent holders to reduce royalties, there may be collusion to fix higher prices for consumers. Technology may become inaccessible to a large number of consumers, hurting the overall public good.
==Patent Trolls==