bro, when you play latest games with all eye candy sets to the maxed possible, you'll know that even the most powerful single GPU also struggling to maintain a playable fps especially when you play at 1080p or higher res. That's where GPU OC taking its part to produce more fps = smooth gameplay. You're right about the life span, but to me as long as we monitor the temps and control it wisely everything should be fine, believe me.. I may not an expert when it comes to overclocking CPU/GPU but from what I read and learnt from OC old timers all over the net, and with some experience in overclocking GTX8800s, HD5850s, 6970s before..i believe as long as we know what we did, the GC should be fine.

Most importantly, we didn't use highest achieved OC for 24/7 but instead just for benching purpose and try to get a highest score possible. same goes with those who OC'ed their CPU using subzero/LN2 etc...they didn't do that for 24/7 usage but just for the score and to exceed the limit+ for fun...just my 10cents though  .