Interesting question. A few years ago for my wedding anniversary, I was elated when my wife agreed to a backpacking trip to Kauai on the Kalalau Trail. After years of hammock camping and being an "expert" I only packed a few fleece sleeping bags -- after all, the overnight temps are going to be in the high 60s, low 70s.
It was cold.
Thankfully, the fleece worked fine once we wrapped ourselves.
In another example, in super-heated Phoenix, Arizona, I had another interesting experience. High 100s in the day, 80s°F at night. No need for an under quilt, right?
I think that our (my) body gets acclimatized to the temperatures and changes in temps. The first few hours are sweltering and no amount of insulation is wanted or needed. However, as the temperature hits its lowest point, my body needs something to stay warm enough.
I think having some sort of insulation is important. It may not need to be much (I found a 1-layer IX works great in hot, dry climates), but it's better to have something than nothing.
I should also mention that the hammock itself can have some insulating properties. The thicker cotton and woven hammocks from South America are great and don't need much but a top blanket to stay warm during most of the temperate year. Parachute nylon, polyester, and ripstop nylon don't have much in the way of insulation, so even in hot temps, the convection can pull enough heat away to make me feel cool, even in hot summer nights.