Re: Redshirt Chekov
Real world - They're going to use a member of the main cast over some no name. As mentioned above.
In universe - Kirk knew Chekov better than any one currently on staff in engineering and wanted someone he actually knew in Engineering during a critical mission. There's no evidence the assignment would've been permanent if Scotty hadn't come back.
"I tell you what you all need, you need to take a thirteenth step, down off your high horse." - Hank Hill, King of the Hill