To avoid unbalancing the game, it would have to cost as much, and takes as much time to build, as an equivalent defensive force based on normal ships. And at that point... why not just build the ships? At least a defensive fleet can be repositioned as needed.
If it cost less and was faster to build than an equivalent fleet, then it just pushes back the first planetary invasions in the game, because everyone would automatically build one as the second thing after a factory. I guess it would be good for players who like to "turtle" and build for a while, before facing a threat. But it might be less fun for those who like a more risky and faster-paced game.
