Seems like a good idea. It would be useful for several things:
* Easily set up the pre-conditions for various challenges.
* Build a trap / obstacle in Creative mode, switch to survival to see how deadly it is.
* Build an awesome structure in Survival mode, switch to Creative so that you can fly around and get screenshots from many angles.
As far as "but cheating" - apparently, some people haven't heard that mods like TMI exist. The only solution to cheating is something called self control.
people WOULD spawn stuff in. it would just start of with small things like, 'Oh I need 1 or 2 more logs to finish my house, But the trees Are pretty far, I could just spawn a few logs into my inventory"
But then it would slowly get bigger. Such as " Oh I need some iron to get myself a new iron armor set. Lets just spawn that stuff in..."
Fact of the matter is, when people have the power to spawn stuff in so easily, they will use it, defeating the purpous of survival.