I work in the Tech Support section of Diskeeper Corporation so I wanted to take a moment to reply. Actually, frequent defragmentation should not wear out a drive as in cases where there is nothing to clean up, nothing should be done to move things around on the volume. Despite that, the resources dedicated to performing a defragmentation when it's not necessary are certainly wasted. In the case of the Windows 7 defragmenter, by default it's set to run only on Wednesday at 1am. Defragmenting only once a week, may not be sufficient though. It's almost certainly not sufficient in the case of very active machines.
Our website contains several whitepapers on the life cycle of machines that are defragmented versus those that aren't. In fact a recent blog post included information about a study done along with Windows IT Pro on that particular subject:
http://www.diskeeper.com/blog/?tag=/hardware+life
In any event, I do feel the act of defragmenting can be a waste in certain cases if it's not done properly or with the right objective in mind. For example, many individuals and other defragmenters mays shoot for achieving 0 fragments. This is all fine and good but if resources are dedicated to addressing all file fragments and the result does not return any marked speed to the machine, this could be considered a waste. Diskeeper's focus is specifically on increasing performance by way of defragmentation and other optimization without also expending needless resources to accomplish the task.
I hope that information helps.