In the early game, Polywall can absorb all the incoming damage as long as your opponent doesn't have 6 attack. When this point is reached though, your opponent will be able to destroy it by simply clicking it because of its Frontline, denying your absorb.
In the late game, Polywalls can be used to defend efficiently if your opponent is attacking for an amount just shy of a multiple of 6; attacks for 11, 17, etc. allow you to absorb 5 damage on your last Polywall. If your opponent's attack is equal to or only one greater than a multiple of six, he will click your Polywalls to destroy them, leaving no or close to no damage to absorb for your final blocker.
Polywall can also be built to save a better absorber by dying in defense, just like you would use standard Walls.
Polywall has the curious effect of actually making multiple Blastforges worse in the set despite being a blue unit. This is because it takes 10 to efficiently spend one on defense in a Polylwall set, so if you have 2 or more Blastforges, you won't be able to spend both of the blues on Poylwalls unless you have 20 or more Drones. Even if you do, this leaves you with no gold to spend the rest of your tech.