@jambalaya, it is a bike…..it will cope…..just like the car does. Security reasons like the OP has stated I can understand. But worrying about the bike getting wet and dirty seems a bit odd.
MPG is fair enough, but if you are driving a newish car doing 60Mpg I would guess that it is not as big an issue as when using a less economical car doing 30Mpg.
Because throwing it in the car is much quicker than attaching a towbar rack or putting it on the roof rack
It takes no time at all to secure the bike to the roof, especially with 591’s. I would say the difference (if any) is minimal.
As the OP says he is putting his bike in the car so he can head straight to the trails so for him it definitely makes sense.