Well, if you make a macro, you can use this code:
Sub SetLineSpacing()
Dim sld As Slide
Dim shp As Shape
For Each sld In Application.ActivePresentation.Slides
For Each shp In sld.Shapes
*If shp.TextFrame.HasText Then*
With shp.TextFrame.TextRange.ParagraphFormat
.LineRuleWithin = msoTrue
.SpaceWithin = 1.1
End With
*End If*
Next
Next
End Sub
But there is one flaw. When you have some text with line spacing 3 (for example) but there is no space to have this kind of line spacing, powerpoint will automatically reduce it, ie. to 2.8. When you run this macro then, the line spacing will become 1.2 or 1.3 or something like that. Only when you run the macro a second time, it will repair this problem. I could not figure out where this problem was coming from though, since debugging the code made it work flawless from first run.