“Chassidim hold one should put on Rabbainu Tam to be Yotze his shita. The Shulcha Aruch agrees that a “chassid” should wear both pairs.”
“The Mishna Berura says it is Gaiva for someone who is not a Chassid to put on Rabbainu Tam Tefilin. (34:3)”
I think some people misunderstood the term “chassid” as used by the Mechaber and Mishna Berura 🙂