I've heard numerous times that installing an OS on a USB key is a bad thing to do, as USBs typically have a certain number of writes before dying, and installing an OS on it will wear it out (unless it's used sporadically for rescue purposes).

Nonetheless, I am very tempted to install some flavour of Linux (Ubuntu or Arch, I haven't decided yet) on a small, transportable, USB Key. My problem is, although you read a lot that it's "bad", you are never told how bad. How long would it last (provided, say, a pc that is 24/7 on)? A month? A year? Five years? Is there recipes to make it last longer? Is there any reason beside weariness that should prevent me from attempting this?
I mean, if it can be calculated, then I could theoretically shield myself by doing regular backups on another key when the deadline gets close (for example).

Notes

• I am not talking of using a USB as a live CD, but actually installing the OS on it.)
• When I say "USB Key", I refer to the little USBs with a flash memory, not an external USB hard drive.

For the curious, my reason is that I work in a lot of different places, on different PCs, and I have a very customized session, with my own WM, my own key bindings, my own scripts, , a selection of plugins for firefox and chrome, etc, and currently I am synchronizing all this through a mix of dropbox, git, and transporting files on USBs, and and it's becoming a chore. It would be much simpler for me to just plug the USB and mount the hard disk of the PC I am using and use it's processing power without actually needing to install any OS on it.