I think it all depends on the political system. In our current FPTP system, in the event that a majority government is elected, I agree with you. However, in a minority/coalition government I think it's completely feasible to have single issue parties, if that single issue is deemed important...
The way I see it, even the most fanatical Green Party voter would accept that there is no way they will get into power in this election, or any in the near future. Yes, they do have some pretty extreme and unrealistic policies, but for me there are also some very important and sensible policies...