Does taking tamoxifen really benefit dcis?

Yes: Tam has 3 rolls-1)w/invasive cancer, it's systemic therapy like chemo 2)in breast conservation (dcis or inv) it reduces the risk of local recur above and beyond surgery & XRT 3)it reduces the chance do a second/new er+ breast cancer elsewhere in the same or opposite breast by 40-50%.

Yes, for ER+ DCIS: The addition of tamoxifen to surgery and radiation therapy for dcis significantly reduced the risk of invasive cancer on the same beast by 40 % and risk for in situ breast cancer on the other breast by 60 percent.