Despite the constant rebuttal to all of these cons:
The truth is as a contractor (if that is what you are) you are a hired gun. Promotion opportunities are nonexistent. You can set career goals, work with a task lead, etc etc to no avail. As a contractor, you are hired to perform a specific function. Once you outgrow that, it is time to leave.
Additionally raises are sub-standard, but thats almost a given anywhere.