Popups in the LightSwitch HTML client is really a nice way to create a popup inside a screen when that Popup will only ever be used inside that screen. In other words, there is no need to create a whole separate Screen and show it as a Dialog since that Popup is only used in one screen.
The power of Visual Studio LightSwitch never ceases to amaze me. Just when you think you've hit a roadblock, a solution seems to pop up after closer investigation. As I say to everyone, "LightSwitch is always the right technology" © :)