The FSX Object Placement Tool is my favourite and fastest way of creating scenery placement files. Unfortunately it does suffer from a few bugs - the most serious one being that a saved scenery file can be easily overwritten with a mission file.
The tool appears to have been designed for creating missions, with the scenery placement tool embedded into it. If you mainly use this for creating scenery xml files, your project needs to be saved Using the 'Save Scenery' button.
A serious problem can arise when exiting the Object Placement Tool - a window opens: 'Mission file has changed. Do you wa