Spider Dragons are creepy crawling critters, but they're useful to have! You don't see any Housefly Dragons flying around, do you? And you won't - not so long as you have a Spider Dragon on your Isle. Spider Dragons also weave beautiful silken webs, which are a delight... at least until you get tangled in one.
You won't ever see a flock of flying Spider Dragons, mostly since they don't fly. They can still move fast in forests, swinging from branch to branch by their web. If you need them to cross a desert though, you might want to call them a taxi.
You might think it's easy to keep an eye on Spider Dragons since they don't fly. You'd be wrong. Once you hatch a Spider Dragon, you'll spend most of your days scrambling up castle steps, trying to keep the dragon off of your tower walls. And if they manage to shoot a web up to a Castle In The Sky... best of luck getting it down.
People don't realize how cuddly Epic Spider Dragons are. Sure, they're big and fearsome, but they're sweethearts to the core. Epic Spider Dragons curl up at the feet of their Caretakers at night, then lick the Caretaker's face to wake them up again in the morning.