I'm a little late to the party ('Private cars – a problem shared'), but this is an important subject and deserves thought and debate.
My starting point is that private cars solve a number of problems for the individual car user – even in a densely populated city – while impacting negatively on all citizens. The trick is to ensure that there are as many good, economically viable and well-integrated alternatives to private car use as possible, while disincentivising the negative effects. Shared cars can be part of a portfolio of such measures.
Are fewer cars and more journeys better than the same number of total journeys made with more cars? Most definitely, yes. A larger number of parked cars for longer periods – in the case of Spurtleshire mostly on public streets – reduces space for other beneficial activities and uses; to name a few: wider pavements, more efficient public transport and city logistics, more attractive, greener cityscapes. Which in turn may encourage more walking (and cycling). In other words: a virtuous cycle.