Rees is the in-game back-up. If there is an injury, he is the guy ready to go in cold. He's got the experience
Kiel is the long term solution. He gets the mop-up duty reps and if there is an injury that keeps Golson out a game, then he would be the one to get the 1st team reps all week and start the next game.
The issue is that, as it has been noted around here, Golson is not a leave early type of QB, so even with 2 years under his belt on campus, he could conceivably be here for 3 more years. Not a bad deal - especially when he started by helping take his team to the championship game in his first year. That makes it very tough for the person who comes in one year behind him.