A rogue is certainly not a waste, they can bring a lot to the table besides just trapmonkeying. But if the only reason you want to play a rogue is for traps then you may be happier with a rogue splash build.
You still need to spend 2 skill points in cross class skills. So that wiz/rogue is spending two points in disable device every level. He has a lot of int so he can afford to do that. It's more difficult to keep cross class skills maxed on lower int characters.