Debating ideas isn’t always a surefire way of defeating them. There are many ideas and people who are unwilling to engage in debates on the same terms as everyone else.
Oftentimes, rather than changing people’s minds you merely put problematic and dangerous figures on a pedestal and give them legitimacy they wouldn't otherwise have.
History has shown that when far-right and dangerous speakers are denied a platform, they lose their legitimacy and respect among the public. Richard Spencer, the prominent American white supremacist, recently acknowledged that no-platforming left him and his followers demoralised and defeated.
Exposing students to dangerous ideas isn't the only way to challenge them. Students and the public can also challenge resist them without giving them the legitimacy of a university platform.