I actually like this approach - it will continue the street wall at roughly the same height as Greenhouse apartments. Hopefully will provide a bit wider sidewalk...
Hopefully they make Huntington Ave. less of a concrete jungle and add a green median like on Seaport Blvd.
I think the Mass Ave Underpass gets in the way of achieving both of these objectives. I have no traffic data at my disposal, but I wonder if it could be eliminated with minimal impact to allow for a surface boulevard there and to make a stronger "gateway" to Back Bay from the S/ SW.