Why shouldn't significant immigrant populations designate particular seats in their legislatures (i.e. Parliament, Congress, etc.) to represent them?
First of all, even “significant immigrant populations” do not have the power to simply “designate particular seats…to represent them.” An immigrant group in the United States, for example, has no legal way in which to get a seat to represent it because seats in the American Congress are based on geographical location. There cannot be a seat or seats for immigrants from the Philippines, for example, because each seat in Congress belongs to a specific location, not to a specific demographic constituency. This means that, even if these immigrant populations should designate seats to represent them, they cannot actually do so.
Now, let us say that immigrant groups had the power to designate a seat to represent them in this way. Let us imagine, for example, that immigrants in Germany formed a party to represent them and that party did well enough to get some seats in the Bundestag under Germany’s system of proportional representation. Why might this be a bad idea? This could be a bad idea because having a party and seats in parliament that specifically represent them might make it harder for immigrants to assimilate and become part of mainstream German society. If immigrants make their own party and have their own designated seats, they will appear to be different and separate from the rest of society. Ethnic Germans will not think of them as real Germans. German political parties will not try as hard to enact policies that will help them because they will see the immigrants as rivals for power. By designating these seats, the immigrants are setting themselves apart from the society in which they live and proclaiming that they are different from that society. This is not a good thing for immigrants to do because it prevents them from assimilating into their new home country.