Here's what "HowStuffWorks" has to say about this.......

"The argument for leaving your computer on all the time is 
that turning it on and off somehow "stresses the computer's 
components". For example, when the CPU chip is running it 
can get quite hot, and when you turn the machine off it cools 
back down. The expansion and contraction from the heat 
probably has some effect on the solder joints holding the 
chip in place, and on the micro-fine details on the chip 
itself. But here's 3 ways to look at that: 

*If it were a significant problem, then machines would be 
failing all the time. In fact, hardware is reliable 
(software is a whole different story, and there is a lot 
to be said for rebooting every day). 
*I don't know a single person who leaves the TV on 24 hours 
a day. TVs contain many of the same components that computers 
do. TVs certainly have no problems being cycled on and off. 
*Most vendors will sell you a 3-year full-replacement 
warrantee for about $150. If you are worried about it, spend 
some of the money you are saving by turning your machine off 
and buy a service contract. Over 3 years you come out way 
ahead!"

Add in the cost of it running none stop, add in that Windows 
gets cranky the longer it's on, and think about the fact that 
if it's on, it's vulnerable to hacking.  IF they weren't 
supposed to be turned off, they wouldn't have put an "OFF" 
switch on the things...AND, as with any electrical item, fire 
is always a real consideration.

Add in also the fact that things change so rapidly. The 
joking commercial showing a guy driving home with his brand 
new comp, and passing a billboard showing the NEXT new one 
has made his obsolete before he even gets it home, is the 
plain truth. Few folks keep their comps for more than a 
few years, it's unlikey they will need to worry about 
replacing components...they'll have already traded their 
old comp in for a brand new one.  As someone told me recently, 
if you find that some switches and things wear out rapidly, 
they were junk to begin with.