i agree with s18000rpms
definately defraging every three days is no good... unless ur usual work is creating and deleting lots of files, then too fragmentation occurs when the disk is getting full, otherwise the OS manages to find enough free space in one go.
there is definately not any noticable drop in performance due to defregmemtation unless in extreme cases. some files perform good when defragmented. for eg ur page file. that is because even to access the page file the OS needs to do a seek operation and if the pagefiles is located all over the disk in sufficient size then it just needs to seek to the nearest block.
For defining impact on HDD life u r not using the right anology and definately defragmentation reduces life of the HDD according the the industry standard anology, MTBF, Mean Time Before Failure. when u think of the life in number of hours(which is true for all machines). like for eg a bulb has a life of 100,000 hrs so if u light it up daily for 10 hrs.. it is
probable that it would burn our earlier then if u light it up one hour daily. Hell!! using this anology you can say that by keeping ur heart beating u r reducing its life
But it is true for all devices, if their use can be turned on/off
as for the effect of heat on HDD read this-
"According to our research, increasing HDD temperature by 5°C has the same effect on reliability as switching from 10% to 100% HDD workload! Each one-degree drop of HDD temperature is equivalent to a 10% increase of HDD service life."
source - *www.digit-life.com/articles2/storage/hddpower.html (long and elaborate)
*www.hardwareanalysis.com/content/article/1540.3/ (short and sweet)
Game players definately should defregment once after a three or four game (un)installs.