A similar topic does not always mean similar content.
By having this feature, double topics can be avoided
This could be a good idea, especially about updates in the Bitcoin space which would have likely been posted in other boards. I however don't think forum plug-in is an area the admins want to explore.
of course the disadvantage is that the users running signature campaigns will have less activity as they'll not answer same thing to the next newbie comes.
Reduction in spam is not a disadvantage. The disadvantage here would be that spammers can still go ahead to post a thread even if they are informed one on the exact topic exists.