This feature exists in a backhanded way:
If someone makes a friend request that you ignore, rather than reject, then that person becomes your "facebook follower" – able to see your activity, but without the ability to participate/respond to it directly.
In the context of facebook, it's extremely off-putting to be the ignored (I speak from personal experience), as it's an indication that someone actively shelved my request, rather than having the guts to cut me loose entirely.
Thus, my overall feeling is this: if Facebook implemented this feature, it would destroy Facebook. Having a lot of "viewers" who can't interact with you as a second base of people to manage subverts and overcomplicates the already-extremely-broken sharing and privacy tools of the site, and would ultimately either push people back towards "friend or not" decision making, or a muddle that resulted in less open and honest sharing, since it would be much harder to know "who is watching".