Segno
CollaideI find it to be more difficult to be pixel perfect at really long distances, and difficult to turn around in case, say a scout, runs around you. It's more convenient to stick with one, but I find it more useful to have several. Similarly, many people use different sensitivities on different classes; sometimes even on different weapons.
FireI guess for people who don't Snipe alot it's somewhat useful, but im 99% sure that if you're sniping alot you can get more consistent with 1 sens.
Anyways, it's there and nobody needs to use it if he doesn't want to, when you're not comfortable with your sniping you can give this a try :)
My desk isn't large enough for me to turn around 90 degrees in half a second on my normal sens.