Waste, but that's why it's created, to point out the quality poster and those abusing the system.
If you check for plagiarism before you give merit, probably big part of those wasted by mistake will be saved.
Yeah, I would advise anyone to do this if you have the time. I tend to dedicate a certain amount of time now to giving merits when I know I have time to check for plagiarism or not strictly plagiarism, but something that has been repeated to death or has been discussed recently. A common trend among newbies is to back a few pages, and just regurgitate a good reply hoping for some merit.