It's generally hard to know what to make of military commanders' requests for more resources. On one hand, they're very close to the situation at hand and thus have a very good knowledge of it. This fact gives them a great deal of credibility in public debate. On the other hand, they're managers, and managers always request more resources whether they need them or not.
This second fact isn't that well appreciated in media coverage of their requests, but it's something that anybody who works in a big organization is very familiar with. If you went to Sor-Hoon Tan, my department chair, and asked her if she'd like to have funds to hire two more philosophy professors, she'd say yes. As would the chair of just about every department in the country. Nobody ever says, "Well, we're making great use of what we have, and while we could do more work with more resources, these positions would really be put to better use in Sociology, so why don't you give this money to them." When something is your job, you focus on doing it, whether it's building the NUS philosophy department or rooting out the Taliban in Afghanistan. Whether the resources you're requesting could be put to better use elsewhere is far from your mind.
It's especially true in a context where there aren't strict metrics for efficient performance and you're going to be evaluated primarily on whether the job gets done, not on how efficient you were with what they gave you. Nobody is going to evaluate McChrystal on whether he accomplished more per life lost or dollar spent than say, Norman Schwarzkopf in the Gulf War, because we don't have any metrics for stuff like that. In fact, requesting resources that can't be used efficiently is probably in McChrystal's interests. The more resources we push into Afghanistan, the bigger a deal Afghanistan is. And the bigger a deal Afghanistan is, the bigger a deal he is if we win. If he pulls it out after making everything look dire, so much the better.
This isn't to accuse the general of doing anything corrupt or particularly self-serving. He's just following the incentives that our system gives him, and that everybody in a position like his pursues. What would be particularly corrupt is if he, say, left his command for a cushy job at some company that profited from his resource requests. (I don't know if generals do that often, though members of Congress certainly do it. Billy Tauzin's name probably deserves to be turned into a word for this.)
There are a lot of ways in which military spending escapes the scrutiny that's applied to domestic budget requests, and this is one of them. If Kathleen Sebelius came out tomorrow and said that the medical and economic situation of ordinary Americans required a health care plan that went above the $1 trillion mark over ten years, it wouldn't have nearly the impact of McChrystal's letter today. Nobody out there has the clout to make the kind of demands on the system that generals do, and they're always going to be demanding more.