Your thread title makes my head hurt.
I would say since they make an immense difference in confort, and aerodynamics, there are few reasons not to. Why wouldn't you?
If you are looking to justify thinking they aren't helpful enough to spend a bit of money on them, then you will be disappointed by this thread after a few hours. Do a search.
Nobody who cycles seriously and rides for over an hour doesn't wear cycling shorts. It doesn't make sense not to!