Well, I was thinking if the cats don't clear you could save them up and mature them later, when there aren't any more on the trail. But if you can't mature them int he cats, then that's the end of that idea. Like I said, I don't think I've ever bothered putting a card in the cats myself, likely because there's no point.
Catacombs don’t clear, but they also never mature, so it’ll usually be a waste to put a New Vampire there. The only way you’ll get any points out of them at that point is if a Hunter goes there at night and you get the Bite result on the die roll.
There may be some odd situations where it is worthwhile, but they’re going to be pretty rare.
One I could think of is if the trail had a full six cards, the last two cities (5th and 6th cards) on the trail had New Vampires AND the Hunters were nowhere near the location corresponding to the 5th location.
In that case, it might be worth catacombing the 6th location:
If you mature it, you’ll get your 2 points and you’ll lose the New Vampire on the 5th location as the trail clears.
If you catacomb it, you’ll have a small chance of still getting points out of it (if a Hunter crosses it at Night and gets bitten) and will still have a good shot at maturing the one right after it.
As others have said the Catacombs can be useful for blocking up key locations and even more so because you can then reinforce those locations by putting an extra encounter token on them (which also slightly increases your chances of drawing another New Vampire).