It does make sense to compare them on a USD basis if you trying to figure out what causes a 10x difference in x-ray cost between the U.S. and India (in USD)
Its the same reason it doesn't make sense to directly compare programmers salaries in San Francisco vs. Pittsburgh. If (made up numbers) 120K$ in SF buys the same lifestyle as 90K$ in Pittsburgh, it means someone making 120K in SF is roughly as wealthy as someone making 90K in Pittsburgh. Certainly saying that the SF programmers make 1.3x more is true, but the more fundamental question is the lifestyle that the income affords you.
And of course a lot of things cost the same regardless of where you live. Manufactured goods cost pretty much the same in India as in the US and sometimes more, because of import duties. Gasoline is significantly more expensive.
The point I was making is that some things cost the same no matter where you live. The Indian dentist who wants to take a foreign vacation, buy an iPhone, or a car needs to spend more, relative to her income, than the American dentist. Purchasing power only holds for things that are labor-bound (mainly services, and goods that are labor-intensive to manufacture) rather than resource-bound.