The other problem with the thalaron weapon is that it only destroys organic matter. The technology of the cubes would be completely intact. And we already saw in Before Dishonor what happens when a Borg cube survives the death of all living drones within it: it mutates into a nanotech Grey Goo juggernaut that swallows every bit of matter thrown at it. True, BD didn't present that as something that happened automatically, and we've seen cases where it didn't ("Collective" on VGR, more or less), but if even one of those depopulated cubes made the same breakthrough, it would've made things far worse. So on every level, it would've been a Bad Idea.
Using super-high-tech weapons against the Borg has always been a bad idea. What happens when the Borg start using it against you?
See, the point I was trying to make that appears to have gotten lost: people keep trying to make the thalaron device out to be something horrible, when in the context of Trek, it's
completely normal. "The Romulans have developed a weapon that can
kill everyone Earth!" "Well, no shit, they've had that for two centuries. And so have we."
There can't even be an argument that the thalaron device is analogous to a neutron bomb--because a neutron leaves the infrastructure relatively intact, thereby very arguably rendering the decision to use such a weapon easier (I would argue it doesn't, as there is a whole, very important field of nuclear strategy involving the destruction of economic recovery resources, but that's neither here nor there). Even by its own egregiously bad technobabbly terms, thalaron radiation would not leave the "infrastructure" intact, because it kills all organic life. That includes trees, and plankton, and bacteria, and any strike capable of killing all the humans on Earth (or Cardassians on Cardassia, etc.) would kill virtually all the life
period. The result is an inert mush of graphite (apparently) and oxygen, hydrogen and nitrogen compounds, that would have to be freaking near-terraformed to be useful again, even if you so much as wanted to breathe on the targeted planet after a few years.
In other words, it's a complete dead-end. It is not any more
useful than judicious use of photon torpedoes or phasers, nor would it (or, I might say,
should it, since for unexamined reasons involving mere body horror it does) evoke any more
terror than weapons portrayed as capable of stripping the crust off the mantle. It's like the Osprey of weapons of mass destruction.
Interestingly, Starfleet and no doubt the other powers
do have access to chemical weapons that can poison a planet completely to specific sapient life forms, as shown in "For the Uniform," which apparently leave the exoteric ecosystem intact, and of course any physical infrastructure worth taking. Where's the outrage there?
Edit: so I guess my complaint is that the thalaron device was mentioned at all, really. Remus is probably mandatory to mention after Nemesis, but at least they're a concept that could be saved; throw the thalaron device in the same scrap pile as antiproton healing rays and omega particles (oh, right...

)