1. In many cases, clients have themselves to blame for "unnecessary" costs. Lots of clients fail to educate themselves on legal issues before they engage with their attorneys, so interactions are lengthier than they need be, and often some work that clients can do themselves (organization and preparation of materials, etc.) gets offloaded to the law firm. Many costly litigation matters arise because clients aren't proactive on the prevention front.
2. Clients often choose the wrong providers. The OP writes:
"Buried in those 200 pages were charges from a law student. It wasn't much. But charging over $200/hr for someone less than halfway through law school is one of those self-inflicted image hits that's tough to stomach."
If you use a large, full-service law firm, you should not be at all surprised that a summer associate billed against a matter. Do some clients push back on summer associate hours? Sure. But if you expect an experienced attorney to perform all of the work for you and you have a low tolerance for summer associate time billed at substantially less than what you would pay for even a mid-level associate, you should probably not be using a large, full-service law firm in the first place.
3. Ultimately, if you do not trust your law firm or are not comfortable with the relationship between the cost of services and the value you receive, you should find a new firm. Unless you're a multinational corporation that purchases ungodly amounts of services from a firm and thus has significant leverage, you absolutely do not want to be known as the stingy client who is always complaining about cost.
Bottom line: if you're not thoughtful about how you consume legal services, scrutinizing your bills is not the most effective use of your time.