In my experience with JProfiler, it's just an all-round slicker experience than the NetBeans profiler. It's easier to get started, easier to interpret the information and, although I haven't measured it, it seems that JProfiler has less of a negative impact on the performance of the application being profiled.
Also, JProfiler integrates nicely with IntelliJ IDEA. I have to use NetBeans to use the NetBeans profiler, which is an inconvenience because I have to manually configure a free-form project to match the layout of my project.
The NetBeans profiler is usable. Unlike IntelliJ, I wouldn't buy a JProfiler licence for my personal projects because, unlike an IDE, it's not a tool you use all day every day. However, for paid work there's no reason not to buy a better tool. It's not expensive compared to the cost of a developer's time.