The question of the value of third party UI libraries has often come up in my career. At one time I was an eager advocate of using a comprehensive UI library. Now my attitude is a bit more pragmatic.
Third party UI libraries are sometimes a boon and sometimes a bane. Even for well seasoned UI frameworks. UI libraries for web applications tend to be particularly troublesome because they so often have a clumsy, heavyweight configuration and API that end up requiring more time to use than they save while the vendors race to win the feature count contest.
For some years, the trend seemed to lean toward the use of UI libraries because building a UI by hand was hard regardless of whether you were working with spaghetti script or with Win32 and MFC. Now Microsoft's toolset supports a much richer set of UI paradigms making the delivery of a great UI much easier right out of the box. The trend seems to be gravitating away from reliance on third party controls toward a more simplified user interface style with a more elegant framework relying more on convention than a feature bloated configuration and black box API.
One area where UI libraries seem to continue to do well, in my view of course, is with Silverlight and WPF (XAML) applications. These UI frameworks and their supporting third party UI control sets seem to have been made for each other. Both rely heavily on the powerful UI declarative language XAML. The rich user interfaces that can be spun up using Silverlight or WPF can be intoxicating.
Ultimately, we need to use UI libraries judiciously, being careful not to overuse them where requirements and the effort needed to learn and configure them are not warranted. One could argue that the limited way in which many third party controls are used in a variety of applications provide us with a bountiful set of examples of the gratuitous overuse of these libraries. Sometimes all you need is a good closet and the kitchen sink that comes with that room builder toolkit is just plain old overkill.