In the past it seems like whatever looked better on the game is what was used. For instance Whitewater and Scared Stiff work well with the plastic all the way as did some of the ramps in the Jpop games, but others like STTNG look better in metal for the returns. Now people are so jaded that "metal is good" and "plastic is cheap" and there is no room for anything else.
Personally I do like plastic ramp with metal return. It looks the best in most games and games with totally metal ramps seem to me to get dirtier faster because of all the metal to metal wear with the initial collision when the ball hits the ramp and makes a tight turn at the top at high speed.
