Never mind. I was under the impression that you couldn't have a button click and event trigger on same control:
<Button x:Name="btnMeasureLength" Content="Length" Click="btnMeasureLength_Click" >
<i:Interaction.Triggers>
<i:EventTrigger EventName="Click">
<esri:MeasureAction . . . . . .
. . . . but I guess you can because it works (for now)
