I've always wanted a Big Dummy just to have a bike called Big Dummy. Maybe I'll just get the decal kit and put it on one of my other bikes.
I can't imagine a scenario where you can use commuting to justify buying a Big Dummy, but if you got one for other purposes you could probably tolerate riding it for 5 miles.
A CX bike is overkill for a 5-mile commute, but for the opposite reason. Five miles isn't really far enough to make the drop bar shine. On a longer ride, the comfort and speed are nice. On a 5-mile ride you barely have time to get into a groove, particularly if a lot of the ride is in an urban area.
I think I'd be tempted to throw some slicks on the 29er. Or, better yet, get a new bike tailored for a ride like this. Maybe get the Big Dummy for hauling and also get a something like a Jamis Coda for the commute.