/Gquit makes your life easier.
It is very helpful that all your toons have access to the guild bank as their own private storage space. Inventory and the normal bank are very small. You can also move items and gold around through the guild bank, so you don't need to wait hours for the arrival of the stuff through the slow mail system.
The goldmaking cycle of sending ore from your miner to JC and the crafted items to your disenchanter will become much faster. Players with multiple accounts may also be flagged for gold-selling, if they mail gold to other accounts. Blizzard is not scanning transfers through the guild bank, AFAIK.
All raid content in the game can be easily pugged so it is not a reason to stay in a guild, IMHO.