If you have default text set, that text should display below the MText formatting bar. Just to be sure we are both on the same page, I made a quick Screencast in which I place two Multileaders using a style that has default text, and then I delete the default text and place another Multileader to show that the overwrite prompt is not displayed.
[Screencast processing is a bit slow today - I will post it later, when it is ready.]
Note that the "edit box" (always disabled, you need to use the "..." button to edit the contents) will display Default Text when there is no default text set. This could be confusing, should your default text ever be set to Default Text, as there would be no visual difference in the dialog. If you are selecting the "..." button and all you get are the MText formatting bar and an empty MText string, then you should have no default text set, and you should not be getting the prompt. Double check to make certain that your default text is not one or two space characters, which may not look much different from no text at all after selecting the "..." button, but in the dialog box, the edit box would be blank .

David Koch
AutoCAD Architecture and Revit User
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