What are you talking about? Only valuable for capitalists? If anything its MORE valuable for socialists. Immigration is a net positive unless your a xenophobe. It brings in new people many of whom are already educated so you get all the benefits without having to pay to educate them, and even the ones who arent as educated still are really hard working people who contribute a lot to society. People who arent go getters dont travel thousands of miles for a better life. The only real challenges presented by a lot of immigration are logistical in nature and socialist economies are better equiped to meet those challenges than capitalist ones. Plus even outside the economic benefits they bring with them diverse cultures and foods which make life more interesting and fun for the people already living there. One of my favorite things about America is all the different cultures and foods you can find there. Immigration only becomes a problem when you treat it like one and start treating the hard working people looking for a better life like criminals and then force them to turn to criminal acts to make ends meet. If you welcome people with open arms and provide them with opportunity then its only ever going to be beneficial to everyone involved.

It's true that immigration can have downsides for workers if a) you are in a neoliberal capitalist shithole, and b) if you have zero solidarity between native and immigrant workers such that immigrants are allowed to be abused and super-exploited as a consequence of their more vulnerable status.

The way to fight things like downwards wage pressure resulting from immigration is through worker solidarity, unionization and other forms of collective action and organization. And yes this is possible to do with people of immigrant background too; after all, in today's day and age, with the technology available, language should be no barrier to organizing together. The only real barrier is racism.

Standing up for the rights of migrants and fighting for the rights of native workers, these are deeply interlinked struggles and the success of one will always positively affect the other. Throwing migrants under the bus in hopes of bettering native labor's position on the other hand is always going to end up having the opposite effect: it empowers capital while it atomizes and fragments the working class.

That's not to say that massive migration is necessarily a good thing. While migration can have very beneficial effects on the receiver country it is often detrimental to the countries of origin which experience brain drain and loss of the most highly trained labor. We also cannot forget that highly unbalanced migration is always caused by something being very wrong and pushing people to feel forced to leave their native lands, something which most people would not freely choose to do under normal circumstances.

Whether it's war, economic deprivation or ecological disasters that are causing this migration, those causes (most of which were brought about by the same neoliberal imperialist global system that also seeks to profit from turning native and migrant labor against each other in order to better exploit both) are what we should be looking at and helping those countries to tackle. We should not merely be shutting people out because that does not fix the problem, it only causes more suffering.

We are Marxists, we have to take a holistic view on these issues and not deal with them in the simplistic, un-dialectical way that a liberal or a fascist would.

It is inherently good for the receiving country actually, more people = more labour power. It is bad for the country losing these people.