There are a few reasons not to use a cover. The first is the gap between the display and the keyboard is very tight! The oils and grit on the top of the key will rub against the LCD panel creating rub marks on the panel. This happens more when you are using a tight sleeve cover to carry your MacBook, MacBook Air or MacBook Pro. Do not use palm rest or keycap covers on a Mac notebook with a Retina display
The second issue is heat! The issue in many MacBooks, MacBook Airs or MacBook Pro’s is heat buildup. There are enough gaps along the edge and in the keyboard its self to allow some air to enter to help cool the logic board.
The newer CPU’s in the MacBooks & MacBook Air’s run cooler they don’t face heat fatigue the other parts of the logic board still can.
As far as the fear of causing harm to the keyboard, Eating and drinking over your keyboard is just not a smart move! So I would avoid it, But we are human! We need nourishment and it always seems to be around a computer! So if you are a klutz then don’t!
Apple has a aggressive keyboard policy for the Butterfly keys keyboards. So while its only for four years I think you’ll likely up grade your system by then. Keyboard Service Program for MacBook, MacBook Air, and MacBook Pro