Basiclly, there is no need to run xscreensaver as root, couse obviusly U should not even be logged in X as root, little less leave your station loged as root for time needed for screen saver to start, as it is the large security issue
Why not run X as root?
Well, as you may or my not already know, X is quite a big "application", so what if it had a minor bug that would let's say overwrite some conf or other files? Running as root would enable the application to do such a thing, while running as ordinary user it would not be able to do so.
Or even worst, if it had a security bug that allows remote logins, somebody could take control of root account directly insteda of plain user account ( which is bad enough already )
Don't use the name of ROOT in vain...
( Quote from kernel README