What developers continually fail to realise is that it's not really their game... it's the players' game. You make the game so other people will play it. If you make your game restrictive or hard to play, people won't play it unless they don't know any better.
That's true for single-player games. I'm a fan of hacking single-player games as much as possible because I love it :) But its a different ballpark when it becomes a multi-player games. If that's the case, then hacking your version of the game can easily make you a cheater, which spoils the fun for everyone else. And in some games where certain achievements are hard-earned, if certain players acquire things through cheating, then it really ruins the game for everyone. (Trust me, I did it to Fire King's Eternal World game repeatedly. :P)
I guess my biggest concern is that this: If the game uses buttons which call verbs which may or may not be accessible at any given moment to the player, then if the player is able to modify their interface to make those buttons available when they shouldn't be, it presents a problem. If nothing else, it makes yet another headache for the developer who has to make sure that all buttons only work exactly when they're supposed to, and there's a good chance something might be overlooked.
So suppose the big fancy online world actually does get released. Now in this game, you can only change your equipment through the inventory window, which means in the head of combat, you can't really change your weapons or your armor without getting pulverized while you're in the equipment window. However, some sneaky player figures out that they can modify their interface to have an input area, and through that input area they can type in the verbs that are used to change equipment. Now they can make themselves a little macro that'll change their equipment instantly without bothering with another window. Suddenly this player has a huge advantage over other players because he can change his equipment instantly whenever he wants...
I'm not sure how much of this is actually plausible since I haven't even fiddled with game interfaces outside the two I made for my games, but I don't like the idea of it being possible. If all else fails, this is something that should DEFINITELY be under the control of the developer, whether to allow interface modifying or not.
Then what's the point of having an interface?