It's important to draw a distinction between the Haliburton contracts and the contracts for private security forces.
In the case of Haliburton, no one disputes that the services provided absolutely were necessary (building bases, the logisitcs of supplying thousands of troops in remote areas, providing essential support services for an army spread out widely over a large geography, etc). While the prase "no-bid contracts" makes good fodder for muckracking journalists and anti-Bush websites, the fact of the matter is that for most of those contracts Haliburton was the only company who even expressed any interest in doing the work, coupled with being qualified to do so (when you consider the logistical challenges involved, there are few enterprises able to do the job).
By contrast it is not at all clear that private security contractors were needed. Companies like Blackwater made slick presentations and convinced the DoD that employing them would actually save the government money, by freeing up regular military personnel from certain mundane duties. And that might have even been technically true, but the tradeoffs (lack of accountability, local backlash, the mercenary mentality, etc) were not worth it.
None of this changes the absolutely stinky appearance created by the use of Haliburton, given Cheney's association there, but still, we should distingush between contracting for necessary and difficult to perform tasks and the use of mercenaries in a delicate, culturally complex situation.