I think this is a terrible idea as suggested, but I think the goal is a good one - making it easier for people to help answer questions by making live demos more accessible. Being able to quickly run some code is a huge benefit.
I really don't like the amount of dependency on external demo sites, jsfiddle in particular. Being able to access SO but have jsfiddle loading like molasses is very frustrating and happens to me all the time.
What I would love to see is an official demo site for Stack Overflow. I think it would solve a lot of problems, especially those concerning the longevity of the content. jsfiddle has no obligation to keep their site active or archive their content forever, although they do say SO makes up about 30% of their trafficSO makes up about 30% of their traffic, and people work around this all the time, posting questions and answers where the relevant content is hidden behind a jsfiddle link.
Until SO gets their own official code demo tools, anything else is just beating around the bush.