Eye Candy
I've never had a customer who was already happy with a stable application that did what was asked demand it "look better". It did the job, and that's what mattered. However, an application must have a certain amount of eye candy to capture the attention of new customers. First impressions count.
With the XP style controls, the latest versions of PowerBuilder can look modern enough on its own. However, there are numerous tools and tricks you can use to make your PB application really stand out.