The thing is with a lot of these parties - Dawn and Jobbik in particular - is that banning them would be more effective than banning say, Thaskin's party (whatever it's called nowadays) which is a personal machine. The appeal with these neo-nazi groups is with their branding. Members march their marches, wave their flags and arrange themselves into cells. That's what the Nazis did - create a flag, gave people a chance to wave it around subsume themselves into nazism. Almost quaint, were it not for the hate they preached. And of course, nobody here (I hope not anyway) would deny that after the war - free speech be damned! - the spectre of nazism had to eliminated entirely. Its organisations were disbanded, their members purged from institutions and its literature burnt. Banning Golden Dawn and its marches, paraphanalia and organised meetings would be no different from that, merely preemptive.
I see no harm in breaking that up - I'm a utilitarian and I see no greater good to society to allowing these organised thugs legitimacy in parliaments; much like I wouldn't care if other organised groups of criminals like the Mafia tried to form a political party and were banned.
Be nice to Dawn:
https://en.wikipedia.org/wiki/Dawn_(Iceland)