No one has fully answered the OP's question (though Sebastian briefly alluded to it), but this thread actually hit on an interesting point of law--Why, once it was clear that McDonald was heading for the Supreme Court, didn't Daley revise the ordinance to make the case moot?
The short answer is that, once a case is in the pipeline, the appellate system will generally see a legal issue through to its conclusion. There are a couple of reasons why this is so:
First, in an instance where a party voluntarily ceases doing something that's claimed to be illegal, that party could resume the illegal conduct once the case has been "mooted" out of the legal system. Here, for instance, had the City of Chicago dropped the ordinance in anticipation of losing in the Supreme Court and had the Supreme Court therefore decided not to rule on the issue, the City would have been free to reinstate the offending ordinance.
Second, the Supreme Court recognizes that there are Constitutional issues that are capable of repetition, but would elude judicial review if the question in controversy was required to be a "live" issue. A well-known example is the Roe v. Wade case. By the time that case got to the Supreme Court, it was well after the nine months of Roe's pregnancy. Simply stated, it was virtually impossible for Roe's case to get through the process of filing a suit, undergoing intermediate appellate review, then being decided by the Supreme Court within the time span of human gestation. Nonetheless, the Court in Roe ruled on the question of the extent to which the state could interfere in abortions, knowing that it was a question of Constitutional law that would be raised in the future by similarly situated litigants.
These doctrines make a lot of common sense, even if you don't agree with the outcome of a particular case. Some scholars have questioned if they meet the Constitutional requirement that the Court rule on "cases and controversies," the theoretical bar being that, if a case has been mooted by the conduct of one party, there is no longer, in fact, a case or controversy. However, if these doctrines were not employed by the Court, important Constitutional questions might never be settled as the result of gamesmanship by the litigants. We might never have seen a Supreme Court decision validating the right to keep and bear arms through the Fourteenth Amendment.
