This is a problem with the original unpatched Supreme Commander Forged Alliance, in which a certain ACU upgrade is unfairly powerful.
In the current (Forged Alliance Forever) version of the game the upgrade has been significantly nerfed and is only rarely seen.
If you use the Restoration Fields, your ACU will heal quicker in addition to any units inside the field's radius. Hence, you can very rapidly heal your ACU at 75 HP/s or 200 HP/s at a dirt-cheap price.
The Restoration Field gives a maximum of 75 HP/s healing, no matter what the unit. An Aeon Galactic Colossus, for instance, will only receive 75 HP/s healing, even though .55% of 99999 HP is about 550 HP/s. (Recall that the Restoration Field heals a unit at .55% of it's total HP/s.) So, too, the ACU recieves a maximum of 75 HP/s healing when under its own Restoration Field upgrade. This does not accumulate with any other HP/s bonuses. For example, if a Seraphim ACU has 60 HP/s healing because of an extremely high veterancy, the Restoration Field will not add 75 HP/s to make 135 HP/s healing. Instead, it adds only 15 to make 75 HP/s, so the Restoration Field upgrade will not go over 75 HP/s.
This means that if a unit is already healing at 75 HP/s, the Restoration Field will have no affect on the unit whatsoever. So take a look at the Nano-Repair System. It gives the ACU an additional 75 HP, so your ACU will be healing at least 85 HP/s, so the Restoration Field will not affect the ACU! This means that when you choose to buy the Restoration Field, you are only paying to heal the units inside the ACU's field, not the ACU itself.
The same thing applies similarily with the Rapid Restoration field, although there's a difference. If you buy the Enhanced Nano-Repair System, your ACU will be healed by at least 160 HP/s. That said, the Rapid Restoration Field heals at 1.1% HP/s, up to a maxiumum of 200 HP/s. This means that unless your ACU is fully veternized (thereby healing at 200 HP/s with the Enhanced Nano-Repair System), the Rapid Restoration Field will affect your ACU by adding from 0 to 40 HP/s healing.
Just like the Cybran ACU's microwave laser or the Aeon's tough shield upgrades, you have to send your Seraphim ACU into battle for the upgrade to be effective. So if you send your ACU with several experimentals toward another ACU or a heavily-armed base, you may be healing your tough units very quickly, but you are putting your ACU in great danger.