MyUberNick says that the [seconds] will be added as an option, there's nothing to suggest the base !mute command will be changed, entering in a massive or non numeric value is handled by input handling in the code, so it's nothing to worry about.
I think it's a welcome addition, though I don't think I'll find a use for it personally. I only mute when there's no concieveable way the player will add anything of real value in text to the game, in which case there's no reason to make it temporary. If someone's just exploding with rage, they usually ragequit or plug, I very rarely if ever see any reason to mute someone to have them cool off, though I probably will soon enough, at which point this command will be useful =)