There are reasons you might not want to do that... The drive will likely spin continuously, and not all enclosures are well-cooled for continuous operation. A greater concern is that the drives are much more fragile when in active use (particularly when the heads are active). Given the exposure of desktop drives to knocks and bumps, even from minor stuff like setting a drink down on the table, it is something to consider. You can see that when you compare the specs of notebook and desktop drives.
Another risk is a pattern of use that has the drive constantly start and stop. Desktop drives don't really like to spin up a Lot. Usually with torrents, the access is more frequent. But someone slowly downloading a single torrent at low bandwidth could create problems, especially since some of the new 2TB drives spin down after only 10 seconds of inactivity.