I like to turn off my pc at night for the following reasons:
1. The pc is a giant night light due to my cooler master haf 932 and the xigmatek dark knight cooler with lights, my g15 and g5 light up too, and the monitor (samsung t240) on standby has a large blinking orange light.
2. the fans (3 230mm fans, 1 120mm, 1 120mm for cpu, vga fan (asus eah4870 dk top 512) aren't quiet...at all.
3. i dont see the point in leaving it on, never noticed any wear at all on any pcs. i still have a compaq from 1998 that runs like the day i bought it (EXTREMELY slowly (was $300 in '98) and the hdd can be heard from every corner of the house, but thats not the point) and i turned that sucker on and off only when i needed to use it.
the only part ive ever had fail on me was two sticks of samsung ram in my old dell, but the hdd is original and still goin. and the only time an hdd failed on me was when i accidentally kicked the usb cable when i was trying to backup my files to it (my heart stopped, i swear) and the clunk of death came louder than the grim reaper knocking on my door.
thats my 2 cents