The statement that you cannot reclaim memory from a leaky application using purge is completely right. The only thing purge will do is to clear the disk buffers. These buffers contain the files you recently accessed (it's a little bit more complicated, but well), so that if you need to access them again, they can be retrieved from memory. OSX uses free memory for those buffers, and releases disk buffer space as soon as more memory is needed.

HOWEVER ... I have an application, actually a plugin in Logic, that checks for the amount of free memory and changes its behavior when that drops below 1GB. It also scans lots of files on starting up. To make it run smoothly, I do clear out memory if free memory goes towards 1GB.