I'd always envisioned the questioned to be this: "Does the lump have the right to ask the potter, 'why'?" I think that's the wrong question.
Lumps of clay are just that, lumps. The question in the text is can God blame us? If he shows mercy to some who are undeserving can he still blame others who are undeserving? It is not, why didn't I get in because I really want in? Why did you reject me when I didn't reject you? That isn't the question asked by a lump.
Some think there are a lot of lost people who want into heaven and are bent out of shape God won't let them in. That isn't the case. I think many people do not want the God of heaven, they don't want to be his fine china. They just don't want other people to be, unless they earn it.
In total, I think this is an argument for the inclusion of the Gentiles and the exclusion of unregenerate Israel. It is God's prerogative to save whoever He wants.