Am still a newbie when it comes to shield emitters; why save low frequency ones if you want to craft a high frequency one, to use Disassemble for parts or...?
The frequency of the shield emitter determines that types of attacks they will be effective at guarding against.
Low frequency emitters absorb more damage from slow attacks with melee and crossbows, while high frequency emitters block more damage from high speed attacks like rifles and energy weapons. Each time a shield absorbs damage it drains from your shield energy. This means that if you have high damage absorption stats on your shield, your shield will sustain fewer hits because it is absorbing all the energy of your enemies attacks.
Sometimes you want your shield to do absorb as much damage as possible because you have no natural resistance. Light armor builds with dodge benefit from a high frequency shield because leather armor does not protect well against energy weapons and you want the shield to absorb all the damage because you don't get hit as often and have no natural resistance.
However, with metal armor, you get hit more often and you have more natural resistance. a low threshold shield with a large energy pool will be able to sustain many hits because the damage reduction will max out at a small number, the damage that gets through will be reduced and easily managed by your metal armor threshold/resistance. Metal armor users benefit the most from high capacity low threshold shields because they don't care about the damage that gets through the shield, but benefit immensely from the slight damage reduction and the ability to sustain more hits.
So the best shield for a metal armor user is a high quality core, an high quality efficiency upgrade, and a low quality low frequency modulator.