Of course companies act for the well being of the company itself, while directors only for theirs, so yeah, insider buying by a company or a director is totally different.
My question is does the data bear it out (presumably the data exists) or is it just an assumption based on the idea that insiders _should_ have a pretty good idea of the value of the company.
http://books.google.com/books?id=iYuh-gfnwb0C&pg=PA361&#...
Basically, it seems that extreme insider buying is a very good sign for the market as a whole, but it's fairly rare. (31.7% returns in one three month period w/ insider buying signal, vs. avg. 4.2% returns in 75 3-month periods w/ insider selling signal)
In terms of individual stocks, it seems that the predictive power of insider buying as a metric is best when mixed with taking into account the company's dividend yield, and they claim that this gives you a 6.2% edge.
For individual stocks the difference is too small for me to feel comfortable.