Combined wood and aluminium windows
I live in Colorado, where it can go from 70F and sunny to 20F and snowing in a day. I have metal-clad wood windows in my house--not a problem. 
Wood window manufacturers try to use wood that minimizes movement. Aluminum has a larger linear coefficient of thermal expansion than wood (12.3 versus 3.0), so I don't believe aluminum would handle temperature variation any better than wood. Perhaps the contractor is worried about other climate effects. I wouldn't do wood that isn't clad in Colorado, for instance, because you would have to refinish it every year--our sun is brutal to wood products.
Good windows are built in a manner to handle temperature fluctuations. Perhaps your climate is very extreme and I just don't know your local building requirements, but the contractor's reasoning sounds a little strange to me.