I think your first question is answered by your second. The main reason for a radically egalitarian Star Trek utopia to have money would be to allocate scarce resources. The shack and the 8-bedroom villa have very different supply and demand curves, so would have very different prices.

There are other problems that get glossed over in a Star Trek utopia, and there are other ways of allocating scarce resources than a market economy, but I don't think your objection really holds.