Pretty cool that they allow players to run their own scripts, and in a full Lua environment no less. I imagine that could be useful in a game with a lot of repetitive tasks.
On the other hand, I did write a handy few handy little addons despite this restriction. For example, when you're doing lots of wheeling and dealing at the auction house (a perfectly legitimate way to make in game money), you tend to receive a huge amount of "mail". When another player buys an item you're selling, you receive the payment via your mailbox. It can be tedious to go through all the messages and take the money or items (mostly a UX issue if you ask me). To get around this, I just added a "take all AH stuff" button to the mailbox UI. There were a few problems, in particular that you'd have to "pump" it, because moving to the "next page" of mailbox items was an asynchronous operation, and the callback wouldn't be able to do the "take item" action.
Users contribute huge numbers of useful addons despite this restriction - for example, auction house addons can remember price data and show you items that are substantially above or below their general market value. The famous "Carbonite" addon made gigantic improvements to the in-game map, most of which were later incorporated into the default interface. And no serious raider would go into battle without a host of combat helpers, DPS meters, and so forth.
To prevent the game from being easily being botted to death, there's not much you can script aside from automating UI interactions. This was a killer feature for the Master Plan mod last expansion, which automated clicking through poorly-designed menus for a poorly-designed mechanic, and it single-handedly broke the economy.
Could you get money by clicking menus? I'm curious
This technique has just been removed from the game, however.
Let's be fair here. It's the Garrison's design that broke the economy - Master Plan just made it worse by turning "a few click every few hours" into "one click every few hours".