This question is something that both designers and customers knock back and forth all the time. So why should you pay for working files?
Well let's be completely transparent first - it's not in our best interest as designers. You can now make changes on your own if you have the software or hire a "cheaper" designer.
But what are the sacrifices? An inexperienced person may end up costing more, if not in money, in time wasted doing and redoing work. You also risk consistency in the quality of work. The designer who created the original work has an strong understanding of you, how you'd like to be perceived and your expectations.
Other then that there is also time that needs to be spent cleaning a file and exporting the files. Everything from ensuring the licensing of fonts and images, creating layers and text styles to make editing easy and ensuring consistency needs to be done.
You have to view artwork like a blue print. If you had invented a new chair and a friend bought one. He loved it so much that he asked if he could have the blue prints for free so he can make his own . . . would you just give it to him?
Design is the creative I.P. of the designer just like a blue pint for a product is the creative I.P. of product creator. We don't legislate and protect in the same way, but just as if you wanted to buy a patent - likewise artwork has a cost.
Would love to hear you opinion.