Aside from a Check Engine Light....
 

...what would be the problem with my driving without the gas cap tightened sufficiently ? Low fuel pressure ? Poor mileage ?

just the gas cap light stays on, and you're allowing gas fumes into the air. fuel pressure will be good, milage will be good, engine runs good.

gasoline evaporates very quickly so yes you would get less MPG.

And very annoying

Plus the awful smell of gasoline that could trigger a fire if someone is smoking near your vehicle.

OK...thanks for these responses to all.

Now if I were thinking that I may have issues with my EVAP canister or solenoid (I'm sure you're familiar with THAT topic) , could simply not tightening the gas cap and allowing any fuel vapors to vent out the tank filler tube vs. needing the "services" of the EVAP system, be an effective way of perhaps ruling out that the EVAP system was in fact malfunctioning ?