Sherlock moved John's chair because he couldn't stand it being empty. On a side note, its not that hard to fold a frame into a chair like that. i do it often. (ahh, the vacant chair; so sad, it hurts my heart!
The tips for how 'not' to do it, are a good idea tool too. Because for every way to commit a crime there is a way for it to be fludged. Or the 'I backed my way into a corner and how is my character going to solve this' kind of issue.