I'm not too sure how I feel about this suggestion. Personally, I don't like it. Now of course, it would be great for subforums such as Contests, Requests & Giveaways, however, on forums that I have seen this implemented on it just equates to low quality posts. A strong equivalent to your suggestion to minimize guest viewers would be to "guest-proof" spoilers so guests would be prompted to register or sign in if trying to click a spoiler. Your suggestion could minimize "leechers," but while doing so it would create a pool of useless posts. Leechers are always going to be a problem, however, there are ways to avoid them when hosting a giveaway.
This feature would be overused in the Gamertags and Social Mediums subforums. Threads within those two subforums generally get well over 100+ thread replies. Next thing you know, you have threads with 50+ just to be shown a "censored" gamertag or social media account. Let me remind you, 50+ useless posts.