I'm not saying Bergeron isn't amazing by himself, but the WOWY stats show him and Marchand are clear amplifiers of each other.
Bergeron without Marchand is 53.94%, Marchand without Bergeron is 52.98%. Together, they're an absurd 61.93% With both off, the Bruins are 50.31.
Historically, Marchand sees a greater drop-off than Bergeron when they are separated. Some of that is who they are down graded to playing with, of course. I don't think anyone would deny they are at their best together. On individual talent alone, there is no reason they both should be on Team Canada's #1 line... but they with Crosby is always clearly the best line. Just insane chemistry between the two.