Yeah, the thing about the high-quality enemy planets is that they are usually very useful to capture rather than destroy, especially if it's an enemy manufacturing center, and especially if it's deep in the heart of enemy territory, and if the AI is working correctly, then the enemy should throw everything they have at your TS, so it wouldn't really be much harder to capture a planet than to destroy its sun.
The times I'd imagine myself using one of these things is when dealing with the low-quality planets my enemy has, that aren't really worth invading and then taking the time and resources to upgrade and defend. Or to use it on high grav/aquatic/radioactive/etc type of planets that I can't colonize (or upgrade if colonized), but that I don't want my enemy to control either. In these cases, making it work more than once would be nice and make it a useful strategy to use. Otherwise, it sounds kind of costly for little reward, IMHO.
I used mine to blow up a total of 4 star systems I believe. I had two ships that traveled around fleeted with it: one large hull with high defense and one small ship with Warp III (Terrans) - total speed 5 pc/turn. I'd cruise in, wait a turn if necessary so everyone had their full movement points then remove the TS from the fleet, move my ships just outside of the system, destroy the star and then move my ships back in and re-fleet. That way the TS was never unprotected in enemy space. I definitely got a lot of bang for the buck, since it totally annihilated 2 enemies and so severely crippled a 3rd that they weren't a credible threat any more (which left me in possession of the game).
As Terror Stars are now, they seem to have two uses - for the hell of it (or because, hey, what else is an Evil ruler to do?) or to strike at an enemy (or a friend/ally about to become an enemy) brutally. If you blow up a few measly little planets that aren't helping your enemy much (unless tons of really high value ships are right there and get destroyed as well) then I'd say you aren't getting much bang for your buck. Wipe out a major production center and you've may be able to drag them back down if they're getting too far ahead.
They aren't weapons to be used if you need those planets that you're destroying more than you need the enemy not to have them

Remember, winning the game doesn't mean that you've conquered the planets - just that no one is left to oppose you. Following a campaign of scorched earth is a valid plan, even if not a great plan if there are future consequences

I'd say there was 1/4 of the map that was a total wasteland when I was finished.
Wonder what would happen if you gifted one to the AI... Probably get destroyed immediately just like lone troop transports.
Anyway, it seems like everyone would like to see them developed further so they are less of a 'novelty' feature.