Good point if the Soviets defend back then Germany can attack whenever it wants.
Absolutely no difficulty breaking the pact.
@JJ:
Anythime

.
Leaving a few cheap single unit speedbumps won't hurt if you have them to spend. Just don't overdo it, and don't do it if it compromises your actual rearward defenseline.
You could just practice Barbarossa scenario a couple of times, or play forward defense solitaire, just for practice.
Or you play a 'cooperative' game online with someone also interested in trying out this particular defense.
If you think about it, rearward isn't anything other than forward, except that the meatgrinding starts a couple of impulses later (and deeper into Russia). Once Germany has moved its army and Luftwaffe to the rearward defense line, the grinding that starts is just the same.
Pulverizing the Red Army or bogging down against it isn't just a matter of the forward or rearward defense. Dice play a huge role, alas, as does the skill of the players, ofc.
I've seen a German assault in Russia getting seriously bogged down with the forward defense and the German player demoralised.