I think that the Purge command is most likely been set aside nowadays. We now have such expanded CPU power, loads of RAM and semi-unlimited storage space for our files. It seems like no one Purges anymore.
Oh yeah, you use it when you are having troubles, but not as a routine part of CAD. I have seen many files that were blotted and needed to loose some “wait”.
So why do you need to Purge from time to time?
1. Reduced file size
2. Avoid potential problem data
3. Cleans out the layer list
4. Purges the following – Blocks, Dimstyles, Layers, LTypes, Plotstyles, Shapes, TextStyles, Mlinestyles, Tablestyles, Regapps or All the above.
Sometimes AutoCAD Purge is not enough. Here are some alternatives to the AutoCAD tool…
SmartPurger  at JTB World
DwgPurge 8.0  by DotSoft
SuperPurge  by Manusoft
M8Tools  for batch Purging