does doing this actually avoid fragmenting my mac's hd?
In a word: no. To elaborate on that: your HD (boot volume) gets 'naturally' fragmented over time by regular use. The fact that you keep one of your activities out of your 'home brew' only stops (actually: minimizes) that one activity from contributing to the fragmentation. In this context you may also want to consider the fact that there are two types of fragmentation, that of the data on disk and that of the disk directory itself.
Btw, this natural fragmentation isn't necessarily all that much of an issue, but for a discussion of the topic see the thread
Why Defrag - By MMT3 in our previous Forums incarnation.
