You don't understand the utility of classes because up until now we've just had 7 of 'em with a very basic specialization(jobs) of each. You can't mix and match to create a versatile skill set if we only have 15 per each class to choose from, especially if many of them are locked to their respective classes. Once we get more classes and thus options, along with the new content Yoshi P is designing for class play in mind, you'll start to see their purpose.

In other words, don't blow your lid just yet just because you haven't seen something yet.