1) They are necessary for Mythic raiders basically, but mostly during the core progression. For example, my entire guild wiped all non-work order based Garrison buildings in order to maximize the amount of Elemental Runes we get per week so that we can get the 710 ring asap. Beyond that, they're in no way necessary... why, the only thing I do are Garrison missions.
But I simply must stress that it's a very good idea to level it up. There's a new trait, Treasure Hunter, which increases gold gain from missions by 100%. It basically means that the Garrison pays back all the money you dished on it within a few days after getting 2 Treasure Hunters to 100 epic, which is silly easy if you have a follower called Harrison Jones, which is gained by doing his 6 dailies.
2) REALLY fun! While many praise Highmaul, the ogre raid, I actually find it underwhelming, but it's entirely possible you'll find it awesome. I find it similar to AQ20/40, Bastion of Twilight, Dragon Soul and Molten Core. It's (essentially) one-way, with quite wild bosses. While they are quite fun, I simply don't fancy the theme of this mostly-ogre ruin.
Blackrock Foundry on the other hand, is friggin awesome! There are 4 wings, and unlike Icecrown Citadel, there are no forced starter bosses. Three wings have three bosses, and the final wing requires all other wings to be beaten, consisting of only one boss (Blackhand). Each end wing boss is actually significantly harder than the first and second bosses of any other wing, and each wing's bosses are quite comparable, so even in Mythic, you can choose which bosses you want to go after first. Theme-wise and on the epicness scale, I find it quite similar in enjoyment and feel to Icecrown Citadel and Black Temple. Dark, fiery, with themed bosses with interesting mechanics that keep you on your toes.
3) People always complain about balance, but really every expansion the balance gets better. Healers are far better balanced today, for example (unlike in MoP, where disc was so op), and while there are quite a few dps specs that are not viable, and outside of RBGs you still won't find consistent victories as a tank, I find that things are pretty ok, but I don't pvp that much. Also, the highly-anticipated Ashran was a monumental failure and Holinka can go get stamped by Hansfrans.
4) Once, a third of my Ashran raid was warriors (about 14 people). I think I don't need to really say anymore!
--
But... to be honest, if I didn't raid, I'd have probably unsubbed. I don't do anything but raid, so if I were to stop, I don't even know what I'd do. But as a raider I love this expansion!