Basically the devmode scenario is this:

There's some things that can "break" the campaign, such as getting in a vehicle after you cheat_bump_possessioned'd a character. You will not be able to get back out, and if the game saves...

So it's kind of a measure to check that people won't break it and then bitch. To be honest, I think it's completely stupid, since by cheating in the first place you are taking the risk yourself. Furthermore, you can just use a stupid trainer that modifies the memory, or you can disable the script through console and cheat anyway.

Dumbass move.