Xenophobes have the right to speak too, as long as they do it by the rules.
If you deny the rights of a sizable part of the population to express themselves just because the majority disagrees with *some* of their ideas, it's not democracy.
Here is an simplified example of the problem :
- 30% of the population are represented by a "far-right xenophobic party", will always vote against laws favoring immigrants
- 60% are represented by a centrist party that have more nuanced ideas
- 10% are represented by a far-left party that always favor immigrants
In all cases, unreasonable demands made by the far-right and far-left party will never pass. Whether or not you give full power to the centrist party, the extremists will never get the required majority.
Now imagine a controversial proposition favoring immigrants (maybe some kind of reverse discrimination). Let's say that 55% of the general population is against it and only 45% wants it. Normally, it should be rejected. However, if you give full power to the centrist party, it will be adopted. It is because the repartition will be as follows :
- 30% are against in and are far-right
- 25% are against it and are centrist
- 35 % are for it and are centrist
- 10 % are for it and are far-left
Giving governing responsibilities to small parties may even protect against extremes in some cases. Consider the following : same situation as before, except that the far-right party, though some clever political moves, manages to grab 25% of the centrist party, reaching 55% and winning the elections. The ideas of the people are the same, it's just that the far-right party is a bit less extreme than before. In a proportional system the remaining 45% are enough to keep it in check. Without it, the 30% original extremists would win as they are more numerous than the 25% centrists turned right-wing.