There are a number of benefits to being in a guild:
1. No forge point cost when trading.
2. Access to additional resources at much cheaper costs than you can produce for yourself, generally faster too.
3. Bonuses from motivating and polishing from people you needn't worry about attacking you
4. Forge point donations when you build a great building
5. Access to great buildings to donate to.
6. Additional people you can motivate and polish to find said great building blue print pieces.
7. Community... if you've got a good guild.
Long and the short, it's a major upside to advancement.
Chat is not yet ready, it's coming though. In the future, despite my dislike of the idea, it appears that guild wars may also be on the horizon.