Pros and cons of buckets on long lines...in no particular order:
Most of it comes down to personal preference, although depending upon terrain you could be forced to use a long line.
Grass fires tend to be easier to fight with the bucket on the belly hook because you can drop faster.
If the only place to dip is surrounded by trees then you have no choice but to put a long line on.
Downwash from the helicopter can fan the flames, so typically, the bigger the helicopter---the long line is preferred, especially on spot drops.
Some say that a line can slow you down, but in the hands of a good VR pilot I have not noticed that much difference.