The problem is when the extremists _are_ one of the two main political parties that the system permits.
See also: The current problems in the USA and the UK.
I also think that democratic parties should have incentives to offer more than just "Vote for us, or the fascists win!" There should be viable choices between _different_ democratic parties, and the voters should not feel that their vote is "lost" even when they are not voting for one of the "Big Two".