* treated as 6 defense because of the pixie
The quoted part... is not very useful. The pixie cannot block. You buy prompt defenders so that the attacker doesn't reach your whatever. The pixie isn't accomplishing that.
The rest you probably know already, but I'll write it down anyway:
It would also be useful to note that some of the prompt defenders have lifespan. Lifespan 1 in particular is countered hard by chill, and soft countered by the attacker being able to transform attack into other ressources (usually defense/absorb or delayed attack).
Saying energy = 0 is a good approximation... until it isn't. Of course, it is usually a good idea to always keep at least one engineer alive to have more granularity in your defense. But personally I don't replace them if it allows me to squeeze an extra attacker. I'm not entirely sure I am right on that account though, it would probably help if I was better able to predict how much attack I would have to defend against two turns later.
And the strength of forcefield and plexocell is that their full cost is delayed: you don't need to replace drones as you consume them. If the game is about to end, Forcefied and Plexo cell are by far the most efficient defenders. And even if that is not the case, not replacing the drone saves you 3 coins, which might very well be enough to build an extra attacker, and 1 attack >>> 1 gold, etc.
Special note on Polywall: because it is frontline, if the opponent works around it, you are nullifying your absorb. So polywall's defense might be as low as 6-(absorb/n_polywalls). For example, if you have one wall and one polywall, and your opponent attacks with 6, the polywall only granted you 4 extra defense.