Is ear protection necessary for General Aviation aircraft?
This study on cabin noise with experiments in a Cessna 172s and a Piper PA-44 got the following results (noise comparisons are mine):


Peak:


Without headset: 101.3 dBA = rock concert (not front row though)
With headset: ~88 dBA* = vacuum cleaner (old bag-style ones, not Dyson...)

Average:


Without headset: 86.26 dBA = heavy traffic (by the side of the road)
With headset: ~73 dBA* = cloth dryer  



Those value are not terrible per-se but on the long term ear damage is very likely.



*The study found that a headset reduces the loudness by 13 dBA in average. The reduction should be even higher with a noise cancelling headset.