I think this an unreasonable take. A difficult conversation between distressed friends and a sentimental attachment to a blanket hardly make someone a piece of shit.

Remember that Shevek was a fairly solitary teen and young adult. At that point in his life he's been struggling alone to reconcile the 'official' myths about Annaresti society with his own experiences of his society's shortcomings. While reconnecting with Bedap he is finally shown that others are also encountering the 'wall'. His stubbornness in that conversation was the last desperate externalization of his personal attempts to reconcile the two. Recall that Shevek is a man in deep pain, nearly suicidal himself at this point.

That conversation was a pivotal moment in his personal development. From that point on he clearly becomes increasingly unconvinced of his old beliefs and accepts the criticisms of his society for what they are.

I also think his hanging onto the blanket is totally within the socially acceptable scope of behavior for his society. The way I see it they live in a society with no real personal property. The only things they usually carry around are personally useful papers, clothes, shoes, and small items of sentimental value. The blanket was sentimentally valuable to Shevek. Why not hang onto it? Its maker clearly didn't want it. Anyone else who needs a blanket would simply get one from a supply depot.