For Minecraft I haven't found much use for it. I could imagine some usefulness if using mods that add a lot of commands, maybe a "Create Waypoint" command for JourneyMap or something along those lines.
For almost all MMO's, I use VA for opening various UI panels. With so many hotkeys these types of games have, calling out the name of a window -- whatever game I'm in -- can always open what I want without having to remember the keybind. Many have a separate Character and Backpack keybinds, so I call out "Full Inventory" and it opens both.
Specifically for WoW, playing as a Hunter, I have a set of hotkeys bound to my number pad and all of my pet commands go there, so I can literally call out orders to my pet and have it attack, Bestial Wrath, go passive and follow ("Pet Heel!"). With my Protection Paladin, I have set my Hands and heals to mouse-over targets, call for "Flash of Light!" and still able to keep moving without losing focus or moving my hands. "Set Focus", "Clear Focus", "Assist" various things like that.
I LOVE VoiceAttack for MMO's