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Why? Because they don't benefit from damage resistances when put on tank or spidertron. (But they do when put on armor.)
This is probably not obvious, but there is a way to make it obvious, using map editor.
Put a flamethrower turret that belongs to enemy, then put a tank in it's range that belongs to player, put a few energy shields on that tank.
Provide that flamethrower turret with light oil and it will attack the tank, it's shields will be depleted in just a few seconds!

In comparison, if you try this without energy shields on that tank, it will only take about 1.75 damage per second, thanks to it's 15/60% fire resistance.
1 minute later, all bacteria in your Gleba base died, but that tank is still there.
3 minutes later, all yumako mash in your Gleba base spoiled, that tank is still there.
4 minutes later, all jelly in your Gleba base spoiled, that tank is still there.
5 minutes later, all nutrients in your Gleba base spoiled, that tank is still there.
10 minutes later, your little 60 espm base finished researching physical projectile damage 6, that tank is still there.
15 minutes later, all pentapod eggs in your Gleba base hatched, causing massive chaos, that tank is still there.
19 minutes later, that tank is finally destroyed.

I think the difference is obvious enough, but you can also try other damage types.
For example, fire a rocket at a tank with 100 shield hitpoints.
Without damage upgrades, a rocket deals 200 explosion damage, and if the tank's 15/70% explosion resistance was applied, 100 hitpoints should be enough to absorb it entirely.
But in fact, the shield absorbs 100 damage first, then the remaining 100 damage is reduced to 25.5 by the tank's resistance, bringing it's health down from 2000 to 1974 .

For spidertron, it's just the same story with different numbers.

Also, I'm surprised that the official wiki didn't mention this at all.

You can equip many shields for even greater protection. You can easily fit four shields for 4 * 150 = 600hp of shield, alongside a fission reactor, batteries, and roboports. A spidertron has an even bigger equipment grid.

And it's not about how long your tank can last before it runs out of HP, its about how shields don't need to be repaired via hand or bot with a repair pack. A shield just magicks its health back.

OneOfTheChips wrote: Wed Jun 10, 2026 4:26 pm You can equip many shields for even greater protection. You can easily fit four shields for 4 * 150 = 600hp of shield, alongside a fission reactor, batteries, and roboports. A spidertron has an even bigger equipment grid.

And it's not about how long your tank can last before it runs out of HP, its about how shields don't need to be repaired via hand or bot with a repair pack. A shield just magicks its health back.

If shields could benefit from damage resistances, then I would agree with your opinions here.
The problem is, when put on tank or spidertron, shields don't benefit from damage resistances.
In this case, 600 shield hitpoints on tank usually only worth 150~200 health. That's not enough for most situations.
And when it comes to saving repair efforts, it's about how long your shields can last before they run out of hitpoints or power.
But without damage resistances, shields can't last very long. Here are some more examples of "bad shield" situation:

• A large biter bites your tank, dealing 30 physical damage. This damage could be reduced to 6 by the tank's 15/60% physical resistance. But your shields have to spend 30 hitpoints to absorb it.
• A medium biter bites for 15 physical damage, that would be reduced to only 0.2 if physical resistance was applied. But your shields just waste 15 hitpoints on it.
• If you ever tried to "steamroll" a spawner with a shielded tank, you'll find it's shields depleted instantly by that impact. Absorbing it entirely requires the same amount of shield hitpoints as the amount of health of that spawner, thanks to the absence of tank's massive 50/80% impact resistance.

Yep. Ever since discovering this I usually just don't bother putting more than one shield module for rocks on tanks. Better to use the space and power for exoskeletons to get hit less. Or for lasers/robots.

I never noticed this but it would certainly explain why my explosive rocket equipped spidertrons tear through their shields like wet paper when they do accidental friendly fire in the midst of a biter nest.