Ah! Let me try to explain. I'll edit the post to make it clearer too! Right now turrets can only shoot to the left, and not behind them (on the right). The dilemma was whether turrets should have the ability to also face right and shoot at the storm when they are successfully shielded and the storm has passed by them.
The concern was less about people spamming shields, but more of "oh my god, these turrets will be half as effective if I don't shield them, so I need to shield them or else they only shoot half the time and get destroyed, wasting my resources spent on turrets!"
This possibly creates a mentality where the player may feel they aren't playing efficiently if they don't spam shields. And if the game encourages spamming shields too much, I think it becomes an obligation rather than fun. So it's more of trying to not make the game feel like shields are a mandatory.
Possibly I might be overthinking it haha.
HOWEVER, I must say I like your idea of capping shields and increasing the limit with each storm. I really like what you proposed because this can tie in to upgrades in the research tree, or even in-game's island scenarios or random card draws where players can choose to increase the shield limit further and make choices of whether to do that or pick something else!