In a sense your question is deeply ironic and I think it's worth underlining that first. Essentially(!) you are defending a realist notion of truth on pragmatic grounds. I just love that.
I could double up on the irony and defend a pragmatic view of truth on realist grounds. I could say that a pragmatic view of truth may not be very useful in everyday life. However it happens to be true, therefore we should pursue it whether we find it helpful or not.
But I won't be so absurd.
A good reason (there may be others) for accepting that all labels are tentative is because history shows that essentialism is the breeding ground of various sorts prejudices, intolerance and injustice. Eugenicists taught that race is a fixed category and that different races are essentially different. Mysogynists think that woman is a fixed category. Simons de Bauvoir on the other hand declared that a woman is not born she is made. Society defines all these things by how it is structured and how we talk about them. And they change over time, as Foucault showed with madness. At one time mad people were believed to have insight. Then they were believed to be evil and locked up. Now we treat it as a medical condition. The same phenomenon constructed in various ways.
We don't know what it is about how society is currently structured that will be subject to revision. Everything is tentative. That doesn't mean we can't believe in things or hold to certain perspectives we find useful. It just means we should be open to the possibility of revision.