Are spiders always bad?

I've seen people being attacked by spiders a few times, and it seems like every time I go and save them, someone gets left behind. Can there ever be a happy ending when you meddle with spiders?

No. There are many positive ends for the Spiders quest.

From perusing the game's data files (this event is called DISTRESS_INFESTATION):

• You can send in your crew to help.
  • This can result in losing one crew member
  • Or it can result in a high-level "stuff" reward (fuel, missiles, drone parts, some scrap - what you get for enemy surrenders)
• If you have an Anti-Personnel Drone, you can use it to gain a medium-level "stuff" reward, and lose a drone part.
• If you have a Boarding Drone, you can use it to gain a low-level "standard" (scrap and two other resources - what you get for destroying a ship) reward, and lose a drone part.
• If you have a Anti-Bio Beam, you can use it to gain a high-level "stuff" reward.

The game data doesn't contain random weights for the events, so I would guess it's 50/50 whether you lose a crew member or gain the reward for sending them in.

Note that even if you have a drone to send in, the rewards are better for risking your crew.

I'd still like to know if it's ever worth sending crew, but I just discovered a "blue" option, which is sending in a Boarding Drone. It seems like the drone has much less to fear from spiders than people do.