And if we break it down into points:
1. It's harder to find a Sphere of Inhibition than a teleport...
2. Instead of Hydras under buffed Archons, we teleport Gorgons and forget about their buff...
3. The ones who arrive are flies and sprites, on the second turn, especially if the Archons are wiped out, then our spellcasters will be faster, and we'll already have four units in the castle...
4. With an explosion, no native Castle hero will kill a Catapult; even a mage isn't always capable of that... especially considering Fortress'