Considering screensavers are a thing of the past and screens turn off when not in use, this package should be optional and not be in the base meta..
One person's thing of the past is another person's necessity. :)
In seriousness, though, the whole point of a meta package is to pull in certain other packages. You could use package.provided (see man portage) to fool portage into thinking that kscreensaver is already installed.
There is an item on the next KDE team meeting about USE flags for meta packages if you are interested.