There are many people who equate tolerance with weakness and immorality. In fact tolerance is a practical necessity as much as it is a moral one. In the world you will be dealing with all sorts of people who are nothing like yourself. You need to learn how to deal with them without you and them killing one another.
Karl Popper said that the tolerant should not tolerate the intolerant, as the intolerant would subvert the tolerant and use it in service of intolerance. Of course when people claim an ideology of tolerance but behave with intolerance, they will always be open to accusations of hypocrisy. There has been vast hypocrisy in such movements as Marxism and political correctness; and there have and always will be people who see such things, however much they get demonized or attacked.
Probably the most weighty practical argument in favour of tolerance is that there are many different people in the world, and for the world to work it is necessary to understand, and work with, all of them. The world is expected to exist for a long time; the question is, in what form. If we have everyone killing one another, that is a stupid way to live. We can do better than that. We need to tolerate people who aren’t like us, then we get to benefit from what they have to give.
Is multiculturalism valid? It is and it isn’t. On the one hand I see no reason to tolerate a culture that thinks it’s right to murder a woman for getting raped or throw sulfuric acid into the face of a child for going to school. However there are many cultures that have valuable things to offer. Much has been learned from the Hindu culture, the Jewish culture, the Chinese culture, the Russian culture. America owes to all of the above.
So if moral arguments don’t work, try practical ones. Once again, tolerance is necessary in a world full of people who aren’t like one another. This is not weakness, and it is not immorality. It is facts of life. And the more people understand this, the better becomes the state of the world.