This could be an important issue in case of a new worm outbreak, if less users are vulnerable this will also lead to less infections. Read the article “Update to the AutoPlay functionality in Windows”: http://support.microsoft.com/kb/971029
So if you choose the uptional download, infections from USB autorun infected computers could be less.
you could also additionally use this: http://www.m2cfg.com/usb_writeblock.htm
edit : usb write block, will it work only until i disconnect it from my pc. in the sense, if i enable write block and remove the usb and put it in other system, can i write into it or not?
It is considered an optional patch. Not exactly aware why it is not considered essential and why it was not launched at the days of the Conficker threat raging. As an essential it could also protect those that are endangered by actions of the unaware. Also Microsoft left a TCP-hole not patched for WK2 and XP, see: http://taosecurity.blogspot.com/2009/09/microsoft-updates-ms09-048-to-show-xp.html
on grounds that were criticised by some security experts,
@nmb
Yes you can toggle with it for all pendrives on a particular machine,