Randall Crane's Urban Planning Research discusses accessibility vs. mobility. He seems to be searching for a definition.
Accessibility is nicely defined as the ease of reaching particular destinations. That can be operationalized (and easily communicated) as how much stuff you can get to in a particular amount of time (e.g. number of jobs within 20 minutes). Our book from the conference is now out. The first of many reports on methods for measuring accessibility will be out soon.
Randall rightly notes that the importance of different things varies for different people. Accessibility measured as above is clearly a supply (or opportunities) measure, and makes no account of demand. No one measure encapsulates the entire economy.
Choices have costs. Increasing acccessibility is not free. Enabling someone to access 101 grocery stores in 30 minutes travel by auto instead of 100 will likely not be noticed unless that grocery is somehow distinct, and valuable, to an individual consumer.