A 'reason' to leave it on all the time?
When you turn a pc off everything cools down - including the metal conductors on components...
The cooling and re-heating of these metal parts will of course cause them to expand and contract ant this can be enough to cause component failure.
A reason to switch off?
If your PC has a sudden power failure, when you try and switch it back on you may find that it will not boot up. The OS can be fried forcing you to re-install your operating system. You may lose data also.
By all means if your PC is not 'doing' anything whilst you sleep/work (EG. Downloading, acting as a server, controlling home automation, acting as a CCTV recorder, recording TV etc.) then switch it off. The chances of losing data are greater than that of component failure. You'll also save a few quid on electricity.
AND if the reason you leave it on all the time is because it takes sooooo long to boot up then why not use Hibernate? Hibernate copies the current state of your session to the HDD in a file and then shuts down the PC. When you switch on it loads this file and you are back where you left off but much sooner