Quite agree that a failsafe to prevent loss of continuous data is vital.
Best way would be to have the X method of closing the window perform a "Minimise to Tray" action, set from a tick box off the Edit/Options menu (might require a "General" tab ?).
Currently, although not as good a method because you lose Multipings own display of the data, you can save every single ping response to a single or several datafiles (one per target if required) and you can then recall that data into another program of your choice. [Multiping does not seem to allow Import of any data even its own captured data.] I learned this tip after emailing Nessoft and was pointed to the Multiping FAQ's which shows how to trigger a "save to file" on every ping. You do need the paid version !