The short trains are necessary on extreme drop angles and very quick transitions. Otherwises, the Gs on the last car/row would be too intense. On a ride like Maverick or Farenheit, if the train were to be 5 or 6 cars long the last car would FLY over the crest of the drop hill. Think about how much negative G you already get in the back row...yeah.
Mess around in NL and you'll see what I mean.
Total Coasters: 145
Top 10 Steel: Maverick, Skyrush, Intimidator 305, Fury 325, Banshee, Millennium Force, Raptor, Superman: The Ride (SFNE), Top Thrill Dragster, Big Bad Wolf
Top 10 Wood: The Beast, El Toro, Phoenix (Knoebels), Boulder Dash, Thunderbolt (KW), Mystic Timbers, Comet (Hershey), Hercules, Shivering Timbers, Jack Rabbit (KW)
Top 5 Hybrid: Twisted Timbers, Goliath (SFGAm), Wicked Cyclone, Great White, Gemini