I hadn't encountered significant slope defense until I started battling at Danville, but Danville is unique in the amount of terrain undulation it has plus it is indoors. Most other battlefields are quite flat, and outdoors. I think both the terrain and lack of sunlight makes slope defense a viable option at Danville.

From my observation, slope defense can be very effective when taken to about 45 degrees with respect to the firing tank - the zone where you are vulnerable from 360 degrees shrinks by about half. However, a tank that is cocked 45 degrees cannot elevate or depress its gun barrel to fire effectively, thus it would seem to be a defensive posture with very little offensive options. I don't employ slope defense for defensive purposes - usually I seek reverse slopes that allow me to fire with minimum exposure of my apple, essentially trying to put the apple in the shadow of my mantlet and hull.

I guess how significant slope defense is, does depend on your playing style - whether you're the patient sniper, or the cavalry bent on flanking and attacking. Or the opponent that you're up against.