Is there an in-game mechanic reason for placing action cards face down during the planning phase
It is to promote communication between players. 

If you and I (try to) use the same lift/elevator in the same turn, one of us will be delayed - and this send all of that player's subsequent plans back a turn!

Likewise for shooting guns, firing missiles, certain energy transfer actions... you can get really messed up if two players "do" the same thing in a single turn.



So what does that have to do with why it is required?
The crux of Space Alert is solving a puzzle in a short amount of time. One aspect of the puzzle is the face-down action cards.
It is common for new players/teams to play with cards face-up. This is, obviously, a "house rule". If you feel that the missions are too challenging, or just want an easier pass at things, then go for it!

Playing with cards face-up allows you to avoid duplication, and make it easier for someone to verify that the mouse is going to be wiggled this phase!