Yes they are. They're worth it for 2 reasons: 1. House of Wolves and Comet 2. The link I attached

Only if you want to get the gun MurMur after completing the short story. Also, if you need ascendant materials then doing the story on lvl 30 is a good way to get the materials.

Depends on if you think it has enough content for your money. The crucible maps are pretty nice, if you play that. There are a few story missions and a strike, and they're pretty full of enemies. Some new items that will of course take awhile to get, and a raid that people say isn't as good as Vault of Glass, but its still another raid. I'm happy with my purchase, others aren't, just know what you're getting before you sign up.