It shaves 8-10 min off the trip to SS from ERGoing under the harbor, possibly?
Somewhere earlier in this thread (and/or in Crazy Transit Pitches) there was a discussion of a similar proposal. It hamstrings the entire works because now three Northside lines (well, four if you count Reading and Haverhill separately, but Reading has its own issues that make it a messy fit for NSRL pair-matching) have to feed off of one (two-track) tunnel. Maybe that problem can be mitigated (especially if the Eastern Route can take service equivalent to half a 4-track NSRL), but if the pair-matching isn't perfect it'd introduce a complication of having a subset of some Southside services not hitting the transfer nodes (or, at least, not doing it at the same place).
The OL doesn't go to Revere. I assume you're adding that as a prerequisite along with a Blue transfer station at Airport (probably feasible) and a Green Line connection to South Station (probably feasible, but expensive). It's a lot of prerequisites to solve the problems with a proposal that...solves a problem that doesn't exist. (If the Eastern Route couldn't be solved with a 4-track CA/T NSRL as has regularly been proposed, that would be one thing, but separating out the Eastern Route just because needs to actually justify itself beyond the cruddy crossings in Chelsea, because blowing all of those to smithereens would cost way less than another cross-harbor tunnel.)
It gives more direct access to Logan from the west and south
It provides an inexpensive way of bringing OL to Everett,Chelsea, and Revere, which sorely needs it
Do you have any data to support your assertion that underwater tunneling is more expensive?