I will say that this always bugged me. I find it beyond impossible to believe that the defenses for Earth would be as lame ("lame" in this case boiling more or less down to "defenses?") as we are shown.
I dunno; it doesn't bother me too much, if starships are indeed the equivalent of SSBNs (and they are seen, several times, doing to an entire planet what a nuclear bomb would do a city, except perhaps even
more thoroughly--nukes don't expose the mantle, for one thing).
I mean, Washington D.C., my capital, is not defended in any realistic fashion whatsoever from a ballistic missile (nor is the city I live in), and I'm fine with that. The defense of the capital is indirectly acheived--any nuclear power that attacks Washington understands that a response in kind is the most likely result of their aggression.
Well, to be fair, the Federation and Starfleet do keep their HQ there. One has to assume that if Earth fell, neither the Federation government nor Starfleet Command would just immediately be thrown into a state of decapitated chaos; there have got to be contingency plans involving government and military infrastructure in place on other worlds (Vulcan or Andor, perhaps).
The big problem with taking out the nerve center, discerned relatively early in the history of the Cold War, is that if even if you had not already committed to a full-scale exchange, you certainly had once you destroyed the capital (and killed the other side's central decisionmakers). The reason being that, in the absence of a universally-recognized authority with whom you can talk, what you have instead are innumerable actors who have access to WMDs whom you can't even contact, let alone negotiate with. Hence most American nuclear war plans, even ones that involved heavy countervalue/economic recovery/your-preferred-euphemism-for-city-killing targeting
still withheld an attack on Moscow, despite its position as the most populous, most heavily industrialized, and hence most valuable target in the USSR.
Nevertheless, still doesn't explain why the Dominion would care. Threatening to destroy Cardassian planets is like threatening to gasoline some anthills. Maybe even less dire, since ants make major contributions to the ecosystem.
Huh, maybe it's less like nuclear war than its World War II precursor of area bombing, so that starships are less SSBNs (one = entire ruined country) and more like heavy bombers (one thousand = seriously damaged city). Actually, this is probably a much better analogy to what is actually seen in DS9, since to get anything done you seem to (depending on plot requirements, I guess) need a fleet to hammer through enemy defenses, and would usually wind up losing much of it even if you were successful.
This might even be overapt. Starfleet can't operate in three-dimensional space because the bomber stream has to stay at a steady altitude (one million km above sea level) to ensure accuracy.
Of course, this still doesn't really seem like it would apply to cloaked ships, though.
That said, even if you have backup plans, your primary HQ going under would have to suck, so Earth at least has in-universe importance. And personally, I'd like to see Earth a little more directly involved in some way in a Trek story or two.
I believe I'd agree--as long as Earth is a
setting, as it is so rarely, instead of a
plot device, as it is so frequently.