It's a feature, which Blizzard has invested a significant amount of time and money into, and has definitely contributed to the game's success. It allows players

It's a feature, which Blizzard has invested a significant amount of time and money into, and has definitely contributed to the game's success. It allows players to modify the game's interface, not just arbitrary game code. Think of it as crowd-sourcing UI improvements from the community. Blizzard is known to take great UI mods and implement them into the game directly. For example, on release I created a mod that let you examine a player's equipped items from any distance. By default, the inspection window would close when a function returned true. This function checked to see if the distance to your target was greater than 5 meters. An easy way to change that behavior was to rebind the function to one that always returned false. It's a natural thing to do in Lua, I think.