Robert Langreth has published a series of articles about personalized cancer treatments in Forbes Magazine (see Part I, Part II, Part III, and/or this summary in GenomeWeb). In a nutshell, the author's main point (in the first article) is that it's very difficult to develop new drugs, and the costs to produce drugs that only help a small proportion of patients may outweigh the benefits for that drug.
I agree with the author in that I don't think it's reasonable to expect to produce an individualized drug for every possible mutation that can cause a disease (such as cancer). However, I think genetic studies can still help improve treatments for several reasons.
First, there are a wide variety of tools that physicians can use to help patients, and I think it is wrong to view this argument from an "all-or-nothing" point of view. Pfizer's response in the third article also criticizes the "all-or-nothing" thinking, although they cite the need for "accumulated modest advances" while I am saying it is good to have more options. For example, the first article mentions the need to develop better surgical methods. I'd like to see more personalized drug treatments as well as new surgical technologies. I imagine there will be certain circumstances where a personalized drug therapy is ideal and certian circumstances where surgery will be necessary. If we reach the point where even 30% of patients can receive personalized drug treatments, then I think that is pretty good.
Second, genetic tools can assist with the diagnosis of existing drugs (or drugs in clinical trails that were not originally designed for individuals with a specific mutation). For example, a drug company may come very close to bringing a drug to the market, but then realize that the drug has severe side-effects for certain individuals. If the individuals with the severe side-effects can be identified ahead of time, then the company can prevent total loss of their research costs by targeting individuals without a particular mutation. Furthermore, scientists can discover new functions for drug candidates (as happened with Viagra), so genetic information may be able to provide researchers with alternative uses for existing drugs (or novel drug candidates).
Finally, it's important to keep in mind that cancer is the 2nd leading cause of death (in the US). I'm sure there is a going to be a point where a mutation in a particular gene (or related pathway) is too rare to warrant developing a personalized treatment, but drugs that can decrease mortality in even 10-20% of patients may still be able to help a large number of people.