Another thread got me thinking about this one, so I figured I'd rouse some debate. =)
Personally, I don't feel that mules constitute cheating. (Disclaimer: I own one mule). There is nothing in the ToS against this, and furthermore you are paying an additional monthly fee for the right to have a mule. The fact that you have 10 character slots, and that every time you add a new character they are placed on the same server your main character is placed on, tells me that SquEnix:
A) Intended for multiple people in one household to have multiple characters, so that those in a household could share Items/Gil.
B) Intended for one person to be able to own multiple characters to experience playing as a member of the various races and genders.
C) Intended for one person to be able to own multiple characters to hold and manage items and Gil for other characters on the same account.
Sure, I think that having 10 characters and gardening with 50 pots on each character is overkill, but if they want to pay the extra money each month, and they want to do all the work involved in that, so be it. Anything that makes playing through this game easier, and is inside the rules, is fine by me.