What are design considerations for grounding an enclosed AC-to-DC converter?
Not sure why, but Dell has its laptops grounded.  This presents more problems than just those related to plugs, since the PC internal ground is also ground for external connectors such as USB, video, etc.  When you connect external usb devices, especially data acquisition and audio equipment, you often find that the response can be different when the laptop is running from batteries vs. plugged into the wall, and these problems go away when you use an isolation plug or transformer.

I always carry an isolation plug (along with my international converter plug kit) whenever I travel. Your charger is most likely double insulated so does not need a ground to be considered safe.