A good reason for avoiding using cores & doing it yourself in pure javascript (some may have seen my steampunk media player or the U-boat widget both of which use javascript to rotate the hands and perform other functions) is that the javascript code is often directly translatable into web widgets.
If the Xwidget engine does ever turn into abandonware (like the yahoo widget engine) then you may find yourself in the position of having a lot of widgets but nowhere to run them (all it takes is a little change in a future version of Windows and Xwidgets could simply stop working). To protect yourself from this, if your working functionality is achieved in javascript code rather than using Xwidget cores then you can create/convert to web widgets that work within the context of a browser.
I have a few widgets that are being converted by Harry Whitfield to run as web widgets at this moment. When I feel confident of the task then I may do the same to some other of my widgets. You may eventually start to see them appearing around the web...
This is the U-boat web widget running within a browser. It isn't just stuck on using Photoshop. It really is running in Firefox. It tells the time and switches from one face to another, the hands all turn and the buttons can be pressed. The rest of it remains to be done but it works.
You can see it operating here:
http://g6auc.me.uk/UBOAT/