Like the figures and vehicles for Wreck-Age.
I think as well as the modelling kick it is also that the cars look better with figures in.
If your using Hot Wheels cars as most of my friends seem to be doing it looks weird having convertibles with no drivers. Also adding figures to pickups look cool.
But on a serious note I think if your using Hot Wheels there are no suitable Bikes or Trikes available, thus the need to find other suppliers in the correct scale.