The decision to evacuate isn't to be made lightly.
Assuming the aircraft has pod underwing engines then the fire should be contained for some time. If you can get confirmation from the FA's and ATC about the state of the fire then that's useful. If the fire is a relatively simple one then the fire fighters will prefer the pax to stay on the aircraft. They are safer there than on a taxi way/run way with fire units driving around and of course, other aircraft around.
An evacuation will result in some injuries as Bealzebub has said.
In short, if the fire can be contained and has fire fighters in attendance then it is best to keep the pax on board where you know where they are. If you can't ascertain the severity of the fire and it's in a position that can't be easily contained, then evacuate.