Limits on third-party spending during election campaigns are absolutely essential to preserve the integrity of campaign spending and donation limits on parties and candidates. The alternative is a free-for-all for millionaires and well-heeled interest groups to spend as they please and thereby gain a disproportionate influence over the democratic process. Such is the case today in the US. 

Incidentally, when Harper was head of the inaptly named National Citizens' Coalition, he launched a constitutional challenge to the third-party spending limits under the Canada Elections Act. It was rejected by the Supreme Court of Canada, which said spending limits are constitutionally permissible as long as the ceilings are reasonable.