Well... exo-shields are one of those elements that have been in there since pre-1.0. And they were added specifically at the request of some players (either alpha players, or some customers, I can't remember which). They were never a unit I was personally interested in at all, but some players love them, is all I can say.
In terms of their costs, they are actually more generous now than they used to be (it was -2 metal and crystal when each harvester gave +6 of one resource, so it was a 2/3 cost). Now it's a flat 50%, and only of the related resource. In recent versions I had simply been ignoring the exo-shield's costs as the harvesters went up in generation, and so they became this wicked-good unit sort of behind my back, heh.
I think they are still legitimately useful on some border planets -- as I noted, half is better than nothing for planets that you are intentionally having as a whipping boy. There are cases in multiplayer games of mine where there's two planets each with 12ish harvester nodes on there, and we can't keep any of them alive. That really would be a good potential place for the exo-shields, they would be cost-positive there.
On the flip side, these are a supremely old unit, and I've never heard anyone saying that they were surprised that they'd been using them wrong, or were tanking their economy because of it, etc. When people see a massive cost like that, it makes them really wary. These are one of those units that are just destined for niche usage, to my mind. When you need them you really need them, but you almost never do, especially the more advanced you get.
And, as a new player, again, I think it's better to get half than nothing if you're struggling that much (to quote It's A Wonderful Life, apparently.