Your own guild, is your own bank as well.
I believe three tabs is the economical point, where another guild is cheaper than a fourth but the third tab was worthwhile; there are 98 slots per tab, so that's close to 300 shared banking slots.

You can deposit stuff and withdraw immediately with another toon, without the mailing delay.

You will easily get 5% bonus experience and 10% flight speed.
Possibly a few ranks beyond that.
I believe guilds that have had five or six boxers level five toons to 85th from 80th, reached level five or so.
Not sure how many teams you have.

A level 10 guild will get you the option of an heirloom cloak.
Which is another +5% or +10% experience for your lower toons.
By the time you have the guild faction to buy it, you'll likely be 85 on the main team already.