Fragmentation has little effect on system performance, with only a couple of exceptions, such as drive too full, or hard drive used for tasks that use the hard drive to a major degree, such as audio recording.
An OS X system that has plenty of free space, the user allows the system to run the periodic maintenance tools (which since 10.4(?) runs without need for the user to remember to run them), and assuming the disk directory is in good shape and the drive itself is mechanically sound - the system does a pretty good job of keeping fragmentation under control. The OS X system does this automatically, without need for a manual tool.