He's a good 4th line pker, the Wings just have too many overpriced guys that have gotten pushed to the 4th line and too many young guys needing to play in the NHL. Miller would be a fine cheap depth add for a contender looking to shore up their PK. Wings would probably trade him for a 6th or 7th or maybe even for a conditional pick as a favor to him.
Miller is an NHLer, Wings just have too many young guys without waiver options on D that are taking up roster spots.
Also it should be noted that GR is so full of vets right now that two have to sit every night, so he doesn't really even have a spot there either.