What are the key reasons for or against using open source systems?

2011 Carter Center Final LimitedElection Observation Report

Public Concerns Regarding theAutomated Election System

Software Architecture

A lack of transparency inherentin the use of voting technologies, in which there is no physical, observablevote count, was also a source of concern. Some critics of the AES called for asystem based entirely on open- source software and wanted automation limited tothe process of vote transmission to canvassing centers, with voting andcounting to be carried out using traditional, manual methods. A manual vote andcount coupled with automated transmission, they contended, would be morecost-effective and would promote public confi- dence in the election; however,efforts to use electoral technologies to decrease the time necessary to conductresults aggregation would potentially be undermined by retaining a manualcount.

Source: https://www.cartercenter.org/resources/pdfs/news/peace_publications/election_reports/philippines-may%202010-elections-finalrpt.pdf