One additional application where classes are essential is in tracking multiple rental properties. While probably not an application for the majority of QW users, it is very useful/powerful for those of us that do need it.
The user assigns a unique class to each property and then appends the proper class to each income/expense transaction. The user also assigns a "Copy #" to each of these classes and QW then itemizes and imports the data into TTax for preparation of the Sched E's - each property then occupies a separate column in the Sched E.
Another case where I found classes very useful was in flagging after- tax contributions to my 401k; e.g., an after-tax transfer from my paycheck to the 401k was categorized "[401k Acct]/AT". I can get a cumulative total of the after-tax contributions by simply running a class report for the class "AT" - essential info for tax implications of retirement distributions.