Honestly, if it were just people bragging about how they took a 3 hour class and got a useless certification, I don't think it would bother me. LinkedIn markets itself as a glorified resume, and it's pretty common for people to fluff up their meaningless certs.

I just feel like there should be somewhat of a boundary between personal and professional life. There are things I would say to my friends that I wouldn't tell anyone in charge of giving me money, at least not directly, and I feel like LinkedIn (purposefully or otherwise) is actively trying to erode that boundary. It's horrible.