Dan,
I'm in full agreement on checking and double checking, but... when teaching students, i have found that this little 'stop gap' is a great way to show it to them, explain about the data being in different spaces.
If they click 'full extent' and their data doesn't line up, they know that they forgot to do something.
Similar with 'Clear Projection'... If one of the layers disappears.... they realize right away that there is an issue.
This gives them a visual, and concrete reasoning of why we must line up our data, rather than it being an abstract concept. Thus far... 10 years in, it has worked, students 'get it' and rarely make Coordinate System/Projection mistakes after the 'Coordinate System and Projections' lab!