As I recall, we both had to figure out the right gain settings on the 60d for our Canon camcorders in the 60d thread. It's that same thing except someone was smart enough to figure out how to use the slate as a reference tone on the 70d.
The slate gets you a fixed reference point on the 70d that you can match to your camera gain settings before you start recording for real so you don't have the camera running say 15db hot relative to the 70d. If you were low setting gain on the 70d XLRs and then corrected it with the camera gain get too high, you'd have a good chance of brickwalling the audio recording on your camera during the actual recording. Adding the slate tone to a recording you're going to use for real without matching gain on the camera wouldn't help you avoid running the risk of brickwalling or having levels set too low, either, on the camera.
In one sense, there are three variable gains: the XLR gain, the camera out gain, and the camera's own gain. By matching the camera out and camera's own gain, hopefully, you have reduced the variables that might mess you up to only the XLR gain when it comes to actual recording.
Setting aside the matter of gain matching for a second, if you're just going to use the audio recorded in the camera and be done, the slate tone is in the way if you actually use it as a slate during the actual recording in my experience because I have to go in and edit it out.
I sometimes run multiple cameras and there's no slate on the ones not connected to the 60d. An old fashioned clapper board would give a better reference on all of them simultaneously for sync purposes. I have yet to use the 70d for an actual video recording.
For the 70d not to pass any signal at the lowest setting is a huge defect, IMO too.