With 13 guilds it feels a little like Bleach... :P
Perks of an alliance include the Alliance Chat and the much larger pool of people to run things with (on top of the Legit Community). Being able to visit allied strongholds also open the doors to being able to buy items from their bases not yet sold in yours and having access to fully manned dragon run flights, more often as various holds get their free flights at different times. My favorite part though is the resource trading. My current behavior is to see what my guild needs and fill that, but any excess goes to allies that require said currencies, and it seems players from the other guilds are doing the same.
The actual alliance listing!
It's really uplifting to see our collective coffers grow via cross guild donation, leading to much faster advancement for all. Because of this I give the "Alliance" part of the alliances update a big thumbs up (obviously your mileage may vary depending on who you ally with)! They also made everything a campaign to help people see the "ladder" of things they need to do, and put some decent rewards behind some. You can read more about this at Shintar's Neverwinter Thoughts! :)