Of course they used delay fuses in the war, and parachutes up to 500lbs.. There are pics and combat films that show it..
Dropping from treetop level was common.. Skeeters and Bostons did what they called "Ramrod" missions into France and low Countries..
Americans in the south pacific, goes without sayin..
The advantage is, with lower angles of fire, you are a harder target for the flack, only appearing over gaps in the trees..
Depends if you have trees, lol.. Also, many heavy AA guns couldn't traverse fast enough to track on you..
Drawback, Actually I think it is much less accurate than a dive bomb attack.. Because the bombs bounce too much, skidding tumbling.. With the delay, you really couldn't say where they would go off.. Like rolling dice.. More like an area attack method than pinpoint..