Shields are a matter of taste. They have some drawbacks, they use fuel, and you can be caught with your shields down, and they offer much worse protection than armor per ton.
Their advantages are regeneration rate and that shields don't leak damage like armor, so it is a trade off. Best way is to look how much protection you get per HS for your armor and compare it to your shields, I think it is 2.5 for delta shields, then you can calculate the time it takes to break even (Regen time*Armor strength/Shield strength), and look if you feel it is worth to strap some shields on. I'd never use shields alone due to their vulnerabilities.
Shields are rather a late game tech, their effectiveness compared to armor improves with tech progression.