Back when I had a 60Hz monitor I overclocked mine to 75Hz and saw a noticeable difference. I did a lot of research into the whole "lifespan reduction" thing and found nothing. I even posted a thread asking for anyone to chime up with a personal experience on lifespan issues or with any related data showing it hurts the monitor. Again, I found nothing to suggest it reduces the lifespan.
That being said, I had never experienced an issue with overclocking my monitors. If you do it, you will see a difference. It's not much, but you'll notice things are a bit smoother and ghosting reduces. As a test, simply grab a window on the desktop and drag it around on the desktop with 60Hz. Then do the same with 75Hz and you'll see what I mean.