Forcefields & You
From Sci Wiki
Forcefields are a pretty mediocre defense. Literally.
(Just go look at the damage table!)
But that's not necessarily a bad thing when you're a defender. It also means that your defense is never rated weak against any attacker's weapon.
At the lowest levels, players may not have enough Reputation to change their weapon's power source to attack a given target. In that case, having something besides a forcefield makes sense. It makes you a less inviting target unless your attacker's weapon is already strong against your defense. Having a defense that will be strong against Physical attack also helps against NPC's.
At higher levels, though, you should assume that an attacker is easily able to change their weapon to be strong against your defense. The only way to foil that is to use the forcefield unless you are actively attacking someone.
Against a forcefield, the attacker will never have better than mediocre strength against you and may look for an easier target. They will definitely have worse odds when attacking you and this may discourage them from attacking you multiple times in any given session.