...I wouldn't have thought twice about using some of these adapters and connectors.
I'm all about the gidgits/gadgets, erector set type of building stuff.
One wouldn't think that they have that much loss by looking at them. ...
Everything you have said is true. However, in most cases the practical difference (in effective communications distance) between "cheap coax, a simple antenna and a few adapters" and "super high end coax, hi gain antenna and no adapters" is not going to matter. On the highway it might amount to 1 quarter, or maybe a half mile. However, other factors such as the physical topography are going to make far more difference and will usually limit your effective range long before you reach the maximum possible distance.
So, sure, do your best to make a nice clean installation and use good materials. But, don't get obsessed. So, if someone gives you an antenna with a BNC connector, and your radio has a standard PL259, and you don't solder. Don't worry, just get the BNC to PL259 adapter.